A Systematic Review of Subclinical Hyperthyroidism Guidelines: a Remarkable Range of Recommendations.

Background Subclinical thyroid diseases are often the subject of debate concerning their clinical significance, the appropriateness of diagnostic testing and possible treatment. This systematic review addresses the variation in international guidelines for subclinical hyperthyroidism, focusing on diagnostic work-up, treatment, and follow-up recommendations. Methods Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, we searched PubMed, Embase, and guideline-specific databases, and included clinical practice guidelines with recommendations on subclinical hyperthyroidism. Guideline recommendations were extracted, and quality assessment was performed using selected questions of the Appraisal of Guidelines for Research and Evaluation (AGREE) II instrument. Results Of the 2,624 records screened, 22 guidelines were included, which were published between 2007 and 2021. Guideline quality was generally intermediate to low. Diagnostic approaches differed substantially, particularly in the extent of recommended testing. Treatment initiation depended on TSH levels, age, and comorbidities, but the level of detail regarding defining precise comorbidities varied. Recommendations for monitoring intervals for follow-up ranged from three to twelve months. Conclusions This review underscores the existing variability in (inter)national guidelines concerning subclinical hyperthyroidism. There is need for clear recommendations in guidelines considering diagnostic work-up, treatment and follow-up of subclinical hyperthyroidism. In order to establish this, future research should focus on determining clear and evidence-based intervention thresholds.

Analysis of Serum Free Thyroxine Concentrations in Healthy Term Neonates Underlines Need for Local and Laboratory-Specific Reference Interval: A Systematic Review and Meta-Analysis of Individual Participant Data.

Background: Initial evaluation of the hypothalamus-pituitary-thyroid axis is done by measuring serum free thyroxine (fT4) and thyrotropin concentrations. For correct interpretation of these measurements, reliable age-specific reference intervals (RIs) are fundamental. Since neonatal fT4 RIs conforming to the Clinical and Laboratory Standards Institute guidelines are not available for all assays, we set out to create literature-based uniform age-specific neonatal fT4 RIs that may be used for every assay. Methods: For meta-analysis of individual participant fT4 concentrations, we systematically searched MEDLINE and Embase (search date December 6, 2023; PROSPERO registration CRD42016041871). We searched for studies reporting fT4 concentrations in healthy term newborns aged 2-27 days, born to mothers without thyroid disease in iodine-sufficient regions. Authors were invited to supply data. Due to standardization differences between assays, data could not be combined for meta-analysis directly, and we attempted to normalize the data using two distinct methods. Results: We obtained 4206 fT4 concentrations from 20 studies that used 13 different assays from 6 manufacturers. First, we set out to normalize fT4 data using the mean and standard deviation of (assay-specific) adult RIs. fT4 concentrations were transformed into Z-scores, assuming a normal distribution. Using a linear mixed-effects model (LMM), we still found a significant difference between fT4 concentration across studies (p < 0.001), after this normalization. As a second approach, we normalized the fT4 concentrations using data from a method/assay comparison study. We used the relationship between the Cobas assay and the other assays as a reference point to convert all values to Cobas values. However, this method also failed to produce consistent results, with significant differences between the normalized data (LMM p < 0.001). Conclusions: We conclude that our attempts at normalizing fT4 assay results were unsuccessful. Confounders related to our unsuccessful analysis may be assay related and/or biological. These findings have significant implications for patient care, since relying on RIs from literature may result in erroneous interpretation of results. Therefore, we strongly recommend to establish local RIs for accurate interpretation of serum fT4 concentrations in neonates.

The Differential Effect of a Shortage of Thyroid Hormone Compared with Knockout of Thyroid Hormone Transporters Mct8 and Mct10 on Murine Macrophage Polarization.

Innate immune cells, including macrophages, are functionally affected by thyroid hormone (TH). Macrophages can undergo phenotypical alterations, shifting between proinflammatory (M1) and immunomodulatory (M2) profiles. Cellular TH concentrations are, among others, determined by TH transporters. To study the effect of TH and TH transporters on macrophage polarization, specific proinflammatory and immunomodulatory markers were analyzed in bone marrow-derived macrophages (BMDMs) depleted of triiodothyronine (T3) and BMDMs with a knockout (KO) of Mct8 and Mct10 and a double KO (dKO) of Mct10/Mct8. Our findings show that T3 is important for M1 polarization, while a lack of T3 stimulates M2 polarization. Mct8 KO BMDMs are unaffected in their T3 responsiveness, but exhibit slight alterations in M2 polarization, while Mct10 KO BMDMs show reduced T3 responsiveness, but unaltered polarization markers. KO of both the Mct8 and Mct10 transporters decreased T3 availability and, contrary to the T3-depleted BMDMs, showed partially increased M1 markers and unaltered M2 markers. These data suggest a role for TH transporters besides transport of TH in BMDMs. This study highlights the complex role of TH transporters in macrophages and provides a new angle on the interaction between the endocrine and immune systems.

Mutant Tbl1x male mice have a short life span and do not breed: unexpected findings.

Humans with the mutation Y509C in transducin beta like 1 X-linked (TBL1X HGNC ID HGNC:11585) have been reported to present with the combination of central congenital hypothyroidism and impaired hearing. TBL1X belongs to the WD40 repeat-containing protein family, is part of NCoR and SMRT corepressor complexes, and thereby involved in thyroid hormone signaling. In order to investigate the effects of the Y509C mutation in TBL1X on cellular thyroid hormone action, we aimed to generate a hemizygous male mouse cohort carrying the Tbl1x Y459C mutation which is equivalent to the human TBL1X Y509C mutation using CRISPR/Cas9 technology. Hemizygous male mice were small at birth and inactive. Their life span (median life span 93 days) was very short compared with heterozygous female mice (survived to >200 days with no welfare issues). About 52% of mice did not survive to weaning (133 mice). Of the remaining 118 mice, only 8 were hemizygous males who were unable to mate whereby it was impossible to generate homozygous female mice. In conclusion, the Tbl1x Y459C mutation in male mice has a marked negative effect on birth weight, survival, and fertility of male mice. The present findings are unexpected as they are in contrast to the mild phenotype in human males carrying the equivalent TBL1X Y509C mutation.

The hypothalamic-pituitary-thyroid axis is intact in male insulin receptor substrate 4 knockout mice.

OBJECTIVE: Loss of function mutations in the insulin receptor substrate 4 (IRS4) gene cause a rare form of X-linked congenital central hypothyroidism in boys and men. Affected individuals show decreased thyroid-stimulation hormone (TSH) secretion. Members of the IRS family canonically act as scaffold proteins between tyrosine kinase receptors and downstream effectors. How loss of IRS4 affects TSH synthesis or secretion is unresolved. We therefore assessed IRS4's role in the hypothalamic-pituitary-thyroid axis of Irs4 knockout mice. METHODS: We generated two global Irs4 knockout mouse lines harboring either two or four base-pair deletions that result in frameshifts and loss of most of the IRS4 protein. RESULTS: Under normal laboratory conditions, Irs4 knockout males did not exhibit impairments in pituitary expression of TSH subunit genes (Tshb or Cga) or in the thyrotropin-releasing hormone (TRH) receptor. Additionally, their serum thyroid hormone, T3 (triiodothyronine) and T4 (thyroxine), and hypothalamic Trh expression levels were normal. When Irs4 knockouts were rendered hypothyroid with a low-iodine diet supplemented with propylthiouracil (PTU) for 3 weeks, their serum TSH increased similarly to wild-type males. CONCLUSIONS: Overall, Irs4 knockout mice do not exhibit central hypothyroidism or otherwise appear to phenocopy IRS4 deficient patients. Compensation by another IRS protein may explain euthyroidism in these animals.

Should we depend on reference intervals from manufacturer package inserts? Comparing TSH and FT4 reference intervals from four manufacturers with results from modern indirect methods and the direct method.

OBJECTIVES: Correct interpretation of thyroid function tests relies on correct reference intervals (RIs) for thyroid-stimulating hormone (TSH) and free thyroxine (FT4). ISO15189 mandates periodic verification of RIs, but laboratories struggle with cost-effective approaches. We investigated whether indirect methods (utilizing historical laboratory data) could replace the direct approach (utilizing healthy reference individuals) and compared results with manufacturer-provided RIs for TSH and FT4. METHODS: We collected historical data (2008-2022) from 13 Dutch laboratories to re-establish RIs by employing indirect methods, TMC (for TSH) and refineR (for FT4). Laboratories used common automated platforms (Roche, Abbott, Beckman or Siemens). Indirect RIs (IRIs) were determined per laboratory per year and clustered per manufacturer (>1.000.000 data points per manufacturer). Direct RIs (DRIs) were established in 125 healthy individuals per platform. RESULTS: TSH IRIs remained robust over the years for all manufacturers. FT4 IRIs proved robust for three manufacturers (Roche, Beckman and Siemens), but the IRI upper reference limit (URL) of Abbott showed a decrease of 2 pmol/L from 2015. Comparison of the IRIs and DRIs for TSH and FT4 showed close agreement using adequate age-stratification. Manufacturer-provided RIs, notably Abbott, Roche and Beckman exhibited inappropriate URLs (overall difference of 0.5-1.0 µIU/mL) for TSH. For FT4, the URLs provided by Roche, Abbott and Siemens were overestimated by 1.5-3.5 pmol/L. CONCLUSIONS: These results underscore the importance of RI verification as manufacturer-provided RIs are often incorrect and RIs may not be robust. Indirect methods offer cost-effective alternatives for laboratory-specific or platform-specific verification of RIs.

Optimizing the Dutch newborn screening for congenital hypothyroidism by incorporating amino acids and acylcarnitines in a machine learning-based model.

Objective: Congenital hypothyroidism (CH) is an inborn thyroid hormone (TH) deficiency mostly caused by thyroidal (primary CH) or hypothalamic/pituitary (central CH) disturbances. Most CH newborn screening (NBS) programs are thyroid-stimulating-hormone (TSH) based, thereby only detecting primary CH. The Dutch NBS is based on measuring total thyroxine (T4) from dried blood spots, aiming to detect primary and central CH at the cost of more false-positive referrals (FPRs) (positive predictive value (PPV) of 21% in 2007-2017). An artificial PPV of 26% was yielded when using a machine learning-based model on the adjusted dataset described based on the Dutch CH NBS. Recently, amino acids (AAs) and acylcarnitines (ACs) have been shown to be associated with TH concentration. We therefore aimed to investigate whether AAs and ACs measured during NBS can contribute to better performance of the CH screening in the Netherlands by using a revised machine learning-based model. Methods: Dutch NBS data between 2007 and 2017 (CH screening results, AAs and ACs) from 1079 FPRs, 515 newborns with primary (431) and central CH (84) and data from 1842 healthy controls were used. A random forest model including these data was developed. Results: The random forest model with an artificial sensitivity of 100% yielded a PPV of 48% and AUROC of 0.99. Besides T4 and TSH, tyrosine, and succinylacetone were the main parameters contributing to the model's performance. Conclusions: The PPV improved significantly (26-48%) by adding several AAs and ACs to our machine learning-based model, suggesting that adding these parameters benefits the current algorithm.

How low can we (reliably) go? A method comparison of thyroid-stimulating hormone assays with a focus on low concentrations.

Objective: International guidelines concerning subclinical hyperthyroidism and thyroid cancer advice absolute cut-off values for aiding clinical decisions in the low range of thyroid-stimulating hormone (TSH) concentrations. As TSH assays are known to be poorly standardized in the normal to high range, we performed a TSH assay method comparison focusing on the low range. Methods: Sixty samples, selected to cover a wide range of TSH concentrations (<0.01 to 120 mIU/L) with oversampling in the lower range (<0.4 mIU/L), were used for the method comparison between three TSH immunoassays (Cobas, Alinity and Atellica). In addition, 20 samples were used to assess the coefficient of variation from duplicate measurements in these three methods. Results: The TSH immunoassays showed standardization differences with a bias of 7-16% for the total range and 1-14% for the low range. This could lead to a different classification of 1.5% of all measured TSH concentrations <0.40 mIU/L measured in our laboratory over the last 6 months, regarding the clinically important cut-off value of TSH = 0.1 mIU/L. As the imprecision of the immunoassays varied from 1.6-5.5%, this could lead to a similar reclassification as the bias between immunoassays. Conclusions: We established the standardization differences of frequently used TSH assays for the total and low concentration ranges. Based on the proportional bias and the imprecision, this effect seems to have limited clinical consequences for the low TSH concentration range. Nevertheless, as guidelines mention absolute TSH values to guide clinical decision-making, caution must be applied when interpreting values close to these cut-offs.

Stability of TSH receptor antibody concentrations and comparability of its immunoassays.

BACKGROUND AND AIMS: Graves' Disease (GD) is an autoimmune form of hyperthyroidism where autoantibodies are directed against the TSH-receptor (TSH-receptor antibodies; TRAb). GD is suspected if TRAb concentrations are above a pre-specified cut-off value. TRAb concentrations are measured using immunoassays. This study aimed to compare the performance of the recently implemented Alinity immunoassay to the KRYPTOR and Cobas TRAb immunoassays. MATERIALS AND METHODS: Left-over serum samples in which TRAb concentrations were measured (KRYPTOR) were used. First, TRAb stability at -20 °C for four to six years and up to five freeze-thaw cycles were assessed. Second, TRAb measurements (n = 436) were repeated using the Alinity and Cobas immunoassay and results (scored as positive/negative based on cut-off value) were compared. RESULTS: TRAb results were stable over five years and up to five freeze-thaw cycles. When comparing immunoassays, 86.2% of the results were similar. Total discrepancy differed between the immunoassays (5.4% Cobas vs Alinity, 8.8% Alinity vs KRYPTOR, 13.3 % Cobas vs KRYPTOR). The KRYPTOR immunoassay showed more negative TRAb results than Cobas and Alinity. CONCLUSION: The Alinity immunoassay showed comparable TRAb results, even though slightly more positive results compared to the KRYPTORand slightly more negative results compared to the Cobas immunoassay were seen.

The role of transducin ß-like 1 X-linked receptor 1 (TBL1XR1) in thyroid hormone metabolism and action in mice.

Transducin ß-like 1 X-linked receptor 1 (TBL1XR1) is a WD40 repeat-containing protein and part of the corepressor complex SMRT/NCoR that binds to the thyroid hormone receptor (TR). We recently described a mutation in TBL1XR1 in patients with Pierpont syndrome. A mouse model bearing this Tbl1xr1 mutation (Tbl1xr1Y446C/Y446C ) displays several aspects of the Pierpont phenotype. Although serum thyroid hormone (TH) concentrations were unremarkable in these mice, tissue TH action might be affected due to the role of TBL1XR1 in the SMRT/NCoR corepressor complex. The aim of the present study was to evaluate tissue TH metabolism and action in a variety of tissues of Tbl1xr1Y446C/Y446C mice. We studied the expression of genes involved in TH metabolism and action in tissues of naïve Tbl1xr1Y446C/Y446C mice and wild type (WT) mice. In addition, we measured deiodinase activity in liver (Dio1 and Dio3), kidney (Dio1 and Dio3) and BAT (Dio2). No striking differences were observed in the liver, hypothalamus, muscle and BAT between Tbl1xr1Y446C/Y446C and WT mice. Pituitary TRα1 mRNA expression was lower in Tbl1xr1Y446C/Y446C mice compared to WT, while the mRNA expression of Tshß and the positively T3-regulated gene Nmb were significantly increased in mutant mice. Interestingly, Mct8 expression was markedly higher in WAT and kidney of mutants, resulting in (subtle) changes in T3-regulated gene expression in both WAT and kidney. In conclusion, mice harboring a mutation in TBL1XR1 display minor changes in cellular TH metabolism and action. TH transport via MCT8 might be affected as the expression is increased in WAT and kidney. The mechanisms involved need to be clarified.

Neonatal screening for primary and central congenital hypothyroidism: is it time to go Dutch?

Thyroid hormone (TH) is indispensable for brain development in utero and during the first 2-3 years of life, and the negative effects of TH deficiency on brain development are irreversible. Detection of TH deficiency early in life by neonatal screening allows early treatment, thereby preventing brain damage. Inborn shortage of TH, also named congenital hypothyroidism (CH), can be the result of defective thyroid gland development or TH synthesis (primary or thyroidal CH (CH-T)). Primary CH is characterized by low blood TH and elevated thyroid-stimulating hormone (TSH) concentrations. Less frequently, CH is due to insufficient stimulation of the thyroid gland because of disturbed hypothalamic or pituitary function (central CH). Central CH is characterized by low TH concentrations, while TSH is normal, low or slightly elevated. Most newborn screening (NBS) programs for CH are primarily TSH based and thereby do not detect central CH. Only a few NBS programs worldwide aim to detect both forms of CH by different strategies. In the Netherlands, we have a unique T4-TSH-thyroxine-binding globulin (TBG) NBS algorithm for CH, which enables the detection of primary and central CH. Although the necessity of central CH detection by NBS is still under debate, it has been shown that most central CH patients have moderate-to-severe hypothyroidism instead of mild and that early detection of central CH by NBS probably improves its clinical outcome and clinical care for central CH patients with multiple pituitary hormone deficiency. We are therefore convinced that detection of central CH by NBS is of utmost importance.

The role of WD40 repeat-containing proteins in endocrine (dys)function.

WD40 repeat-containing proteins play a key role in many cellular functions including signal transduction, protein degradation, and apoptosis. The WD40 domain is highly conserved, and its typical structure is a ß-propeller consisting of 4-8 blades which probably serves as a scaffold for protein-protein interaction. Some WD40 repeat-containing proteins form part of the corepressor complex of nuclear hormone receptors, a family of ligand-dependent transcription factors that play a central role in the regulation of gene transcription. This explains their involvement in endocrine physiology and pathology. In the present review, we first touch upon the structure of WD40 repeat-containing proteins. Next, we describe our current understanding of the role of WD40 domain-containing proteins in nuclear receptor signaling, e.g., as corepressor or coactivator. In the final part of this review, we focus on WD40 domain-containing proteins that are associated with endocrine pathologies. These pathologies vary from isolated dysfunction of one endocrine axis, e.g., congenital isolated central hypothyroidism, to more complex congenital syndromes comprising endocrine phenotypes, such as the Triple-A syndrome.

Lower accuracy of testosterone, cortisol, and free T4 measurements using automated immunoassays in people undergoing hemodialysis.

OBJECTIVES: Hormone measurements using automated immunoassays (IAs) can be affected by the sample matrix. Liquid chromatography tandem-mass spectrometry (LC-MS/MS) is less affected by these matrix effects. In clinical laboratories, testosterone, cortisol and, free thyroxine (FT4) are often measured using IAs. Renal failure alters serum composition in blood samples from people undergoing hemodialysis (HDp) and have, therefore, a complex serum constitution compared to healthy controls (HC). The goal of this study was to investigate the accuracy of testosterone, cortisol, and FT4 measurements in samples of HDp and to get more insight in the interfering factors. METHODS: Thirty serum samples from HDp and HC were collected to measure testosterone, cortisol, and FT4 using a well standardized isotope dilution (ID)-LC-MS/MS method and 5 commercially available automated IAs (Alinity, Atellica, Cobas, Lumipulse, UniCel DXI). Method comparisons between LC-MS/MS and IAs were performed using both HDp and HC samples. RESULTS: Average bias from the LC-MS/MS was for testosterone, cortisol, and FT4 immunoassays respectively up to 92, 7-47 and 16-27% more in HDp than in HC samples and was IA dependent. FT4 IA results were falsely decreased in HDp samples, whereas cortisol and testosterone concentrations in females were predominantly falsely increased. Correlation coefficients between LC-MS/MS and IA results were lower in HDp compared to HC samples. CONCLUSIONS: Several IAs for testosterone (in women), cortisol, and FT4 are less reliable in the altered serum matrix of samples of HDp than in HC. Medical and laboratory specialists should be aware of these pitfalls in this specific population.

Machine learning to improve false-positive results in the Dutch newborn screening for congenital hypothyroidism.

OBJECTIVE: The Dutch Congenital hypothyroidism (CH) Newborn Screening (NBS) algorithm for thyroidal and central congenital hypothyroidism (CH-T and CH-C, respectively) is primarily based on determination of thyroxine (T4) concentrations in dried blood spots, followed by thyroid-stimulating hormone (TSH) and thyroxine-binding globulin (TBG) measurements enabling detection of both CH-T and CH-C, with a positive predictive value (PPV) of 21%. A calculated T4/TBG ratio serves as an indirect measure for free T4. The aim of this study is to investigate whether machine learning techniques can help to improve the PPV of the algorithm without missing the positive cases that should have been detected with the current algorithm. DESIGN & METHODS: NBS data and parameters of CH patients and false-positive referrals in the period 2007-2017 and of a healthy reference population were included in the study. A random forest model was trained and tested using a stratified split and improved using synthetic minority oversampling technique (SMOTE). NBS data of 4668 newborns were included, containing 458 CH-T and 82 CH-C patients, 2332 false-positive referrals and 1670 healthy newborns. RESULTS: Variables determining identification of CH were (in order of importance) TSH, T4/TBG ratio, gestational age, TBG, T4 and age at NBS sampling. In a Receiver-Operating Characteristic (ROC) analysis on the test set, current sensitivity could be maintained, while increasing the PPV to 26%. CONCLUSIONS: Machine learning techniques have the potential to improve the PPV of the Dutch CH NBS. However, improved detection of currently missed cases is only possible with new, better predictors of especially CH-C and a better registration and inclusion of these cases in future models.

Role of hepatic deiodinases in thyroid hormone homeostasis and liver metabolism, inflammation, and fibrosis.

Thyroid hormones play an essential role in regulating whole-body homeostasis. Deiodinases are known to convert thyroid hormone from the prohormone thyroxine (T4) to the bioactive hormone tri-iodothyronine (T3) and convert both T4 and T3 toward their inactive metabolites 3,3',5'-tri-iodothyronine (rT3) and 3,3'-di-iodothyronine (3,3'-T2). Deiodinases are thus important for the regulation of intracellular thyroid hormone concentrations. This is known to be crucial both during development and adult life in regulating thyroid hormone-related gene transcription. This review discusses the importance of liver deiodinases in determining serum and liver thyroid hormone concentrations, liver metabolism and liver disease.

Divergent Thyroid Hormone Levels in Plasma and Left Ventricle of the Heart in Compensated and Decompensated Cardiac Hypertrophy Induced by Chronic Adrenergic Stimulation in Mice.

Chronic hemodynamic overload of the heart induces ventricular hypertrophy that may be either compensatory or progress to decompensation and heart failure. The gradual impairment of ventricular function is, at least in part, the result of a reduction of cardiac thyroid-hormone (TH) action. Here, we examined the proposed roles of increased cardiac expression of the TH-inactivating enzyme deiodinase type 3 (D3) and reduced plasma TH levels in diminishing cardiac TH levels. Using minipumps, mice were infused for one and two weeks with isoproterenol (ISO) alone or in combination with phenylephrine (PE). Remodeling of the heart induced by these adrenergic agonists was assessed by echocardiography. Left ventricular (LV) tissue and plasma TH levels (T4 and T3) were determined using liquid chromatography-tandem mass spectrometry. LV D3 activity was determined by conversion of radiolabeled substrate and quantification following HPLC. The results show that ISO induced compensated LV hypertrophy with maintained cardiac output. Plasma levels of T4 and T3 remained normal, but LV hormone levels were reduced by approximately 30% after two weeks, while LV D3 activity was not significantly increased. ISO + PE induced decompensated LV hypertrophy with diminished cardiac output. Plasma levels of T4 and T3 were substantially reduced after one and two weeks, together with a more than 50% reduction of hormone levels in the LV. D3 activity was increased after one week and returned to control levels after two weeks. These data show for the first time that relative to controls, decompensated LV hypertrophy with diminished cardiac output is associated with a greater reduction of cardiac TH levels than compensated hypertrophy with maintained cardiac output. LV D3 activity is unlikely to account for these reductions after two weeks in either condition. Whereas the mechanism of the mild reduction in compensated hypertrophy is unclear, changes in systemic TH homeostasis appear to determine the marked drop in LV TH levels and associated impairment of ventricular function in decompensated hypertrophy.

Increased fT4 concentrations in patients using levothyroxine without complete suppression of TSH.

Introduction: In our hospital, physicians noticed high free thyroxine (fT4) concentrations without complete suppression of thyroid-stimulating hormone (TSH) in blood samples of patients at the outpatient clinic, which appeared to occur more often following the introduction of a new fT4 immunoassay. This discordance may be explained by incorrect reference intervals, analytical issues, or patient-related factors. We aimed to establish the contribution of the possible factors involved. Methods: Reference intervals of both fT4 immunoassays were re-evaluated using blood samples of healthy volunteers and the new immunoassay's performance was assessed using internal quality controls and external quality rounds. The frequency of discordant fT4 and TSH pairings obtained from laboratory requests were retrospectively analysed using a Delfia (n = 3174) and Cobas cohort (n = 3408). Last, a literature search assessed whether the time of blood draw and the time of levothyroxine (L-T4) ingestion may contribute to higher fT4 concentrations in L-T4 users. Results: The original reference intervals of both fT4 immunoassays were confirmed and no evidence for analytical problems was found. The Delfia (n = 176, 5.5%) and Cobas cohorts (n = 295, 8.7%) showed comparable frequencies of discordance. Interestingly, 72-81% of the discordant results belonged to L-T4 users. Literature indicated the time of blood withdrawal of L-T4 users and, therefore, the time of L-T4 intake as possible explanations. Conclusions: High fT4 without suppressed TSH concentrations can mainly be explained by L-T4 intake. Physicians and laboratory specialists should be aware of this phenomenon to avoid questioning the assay's performance or unnecessarily adapting the L-T4 dose in patients.

Neuronal Blockade of Thyroid Hormone Signaling Increases Sensitivity to Diet-Induced Obesity in Adult Male Mice.

Thyroid hormone increases energy expenditure. Its action is mediated by TR, nuclear receptors present in peripheral tissues and in the central nervous system, particularly in hypothalamic neurons. Here, we address the importance of thyroid hormone signaling in neurons, in general for the regulation of energy expenditure. We generated mice devoid of functional TR in neurons using the Cre/LoxP system. In hypothalamus, which is the center for metabolic regulation, mutations were present in 20% to 42% of the neurons. Phenotyping was performed under physiological conditions that trigger adaptive thermogenesis: cold and high-fat diet (HFD) feeding. Mutant mice displayed impaired thermogenic potential in brown and inguinal white adipose tissues and were more prone to diet-induced obesity. They showed a decreased energy expenditure on chow diet and gained more weight on HFD. This higher sensitivity to obesity disappeared at thermoneutrality. Concomitantly, the AMPK pathway was activated in the ventromedial hypothalamus of the mutants as compared with the controls. In agreement, sympathetic nervous system (SNS) output, visualized by tyrosine hydroxylase expression, was lower in the brown adipose tissue of the mutants. In contrast, absence of TR signaling in the mutants did not affect their ability to respond to cold exposure. This study provides the first genetic evidence that thyroid hormone signaling exerts a significant influence in neurons to stimulate energy expenditure in some physiological context of adaptive thermogenesis. TR function in neurons to limit weight gain in response to HFD and this effect is associated with a potentiation of SNS output.

Hypothyroidism: The difficulty in attributing symptoms to their underlying cause.

Common symptoms of overt hypothyroidism are non-specific and include fatigue, lethargy, and dry skin. Although the diagnosis is considered to be straightforward, no single symptom can be used to identify patients with overt hypothyroidism, while many patients with subclinical hypothyroidism are asymptomatic. A large population-based study on the spectrum of symptoms in subclinical hypothyroidism showed similar rates of thyroid disease-related symptoms compared with euthyroid subjects, while the TSH concentration had no impact on symptom score. Together, these findings make it challenging to attribute symptoms to their underlying cause. This is also true in the case of unexplained persistent symptoms in levothyroxine-treated patients. Although generally considered a life-long replacement therapy, successful thyroid hormone discontinuation resulting in euthyroidism has been reported in approximately one third of patients. Thus, we overtreat patients with (subclinical) hypothyroidism, highlighting the importance of reliable diagnostic criteria. The diagnostic process, including the implementation of robust TSH and FT4 reference intervals, is especially challenging in specific situations including aging, pregnancy, non-thyroidal illness, and central hypothyroidism. There is a clear need for improved adherence to current guidelines from scientific societies and for willingness to manage symptoms without a clear pathological correlate, especially in the case of mild TSH elevations. This review will highlight recent literature on this topic and offers some practice points.