The importance of iodine in public health.

Iodine (I) deficiency has been known for more than a century and is known to cause cretinism at the extreme end of the spectrum but also, importantly, impaired development and neurocognition in areas of mild deficiency. The WHO has indicated that median urinary iodine of 100-199 µg/l in a population is regarded as indicative of an adequate iodine intake. The understanding of the spectrum of iodine deficiency disorders led to the formation of The International Council for the Control of Iodine Deficiency Disorders which has promulgated the use of household iodized salt and the use of such salt in food processing and manufacture. Iodine deficiency is particularly important in pregnancy as the fetus relies on maternal thyroxine (T4) exclusively during the first 14 weeks and also throughout gestation. As this hormone is critical to brain and nervous system maturation, low maternal T4 results in low child intelligence quotient. The recommendation for I intake in pregnancy is 250 µg/day to prevent fetal and child brain function impairment. During the past 25 years, the number of countries with I deficiency has reduced to 32; these still include many European developed countries. Sustainability of adequate iodine status must be achieved by continuous monitoring and where this has not been performed I deficiency has often recurred. More randomized controlled trials of iodine supplementation in pregnancy are required in mild iodine-deficient areas to inform public health strategy and subsequent government action on suitable provision of iodine to the population at risk.

Factors associated with moderate neonatal hyperthyrotropinemia.

BACKGROUND: Maternal iodine deficiency is related to high neonatal thyroid-stimulating hormone (TSH) values, with the threshold of 5 mIU/L recommended as an indicator of iodine nutrition status. The objective of this study was to analyse possible risk factors for increased TSH that could distort its validity as a marker of iodine status. The clinical relevance of this research question is that if the factors associated with iodine deficiency are known, iodine supplementation can be introduced in risk groups, both during pregnancy and in newborns. METHODS: A case-control study was carried out in a sample of 46,622 newborns in 2002-2015 in Spain. Of these, 45,326 had a neonatal TSH value ≥5 mIU/L. The main variable was having TSH ≥5 mIU/L and the secondary variables were: sex, gestational age, day of sample extraction and maternal origin. Associated factors were analysed through a logistic regression model, calculating the odds ratio (OR). RESULTS: The factors associated with this outcome were: male sex (OR = 1.34, 95% CI: 1.20-1.50, p<0.001), originating from an Asian/Oceanic country (OR = 0.80, 95% CI: 0.54-1.20, p = 0.536) or Europe (OR = 0.80, 95% CI: 0.66-0.96, p = 0.285) (including Spain, OR = 1) [p<0.001 for America (OR = 0.54, 95% CI: 0.44-0.68) and p = 0.025 for Africa (OR = 0.78, 95% CI: 0.62-0.97)] and fewer days from birth to sampling (OR = 0.80, 95% CI: 0.77-0.82, p<0.001). CONCLUSIONS: The risk of high neonatal TSH without congenital hypothyroidism is higher in males, decreases with a greater number of days from birth to extraction, and is dependent on maternal ethnicity but not on gestational age.

Role of serum carrier proteins in the peripheral metabolism and tissue distribution of thyroid hormones in familial dysalbuminemic hyperthyroxinemia and congenital elevation of thyroxine-binding globulin.

To investigate the role of thyroxine-binding globulin (TBG) and albumin in the availability of thyroid hormones to peripheral tissues, comprehensive kinetic studies of thyroxine (T4) and triiodothyronine (T3) were carried out in eight subjects with familial dysalbuminemic hyperthyroxinemia (FDH), in four subjects with inherited TBG excess, and in 15 normals. In high-TBG subjects, the reduction of T4 and T3 plasma clearance rates (by 51% and 54%, respectively) was associated with normal daily productions; T4 and T3 distribution volumes were significantly reduced. In FDH subjects T4 clearance was less reduced (by 31%) than in high TBG; consequently T4 production rate was significantly increased (by 42%); T4 and T3 distribution volumes and T3 clearance rate were unchanged. Increased T3 peripheral production in FDH (by 24%) indicates that T4 bound to abnormal albumin is more available to tissues than T4 carried by TBG, thus suggesting an important role of albumin in T4 availability to the periphery.

Thyroxine binding in a TTR Met 119 kindred.

Recently, a transthyretin variant, TTR Met 119, in which methionine substitutes for threonine 119, a component of the protein's iodothyronine binding site, was identified in individuals with transient euthyroid hyperthyroxinemia. Healthy carriers of Met 119 have normal serum thyroid hormone concentrations, but two studies of Met 119 carriers have differed as to whether T4 binding to TTR is increased. An additional kindred has been identified by hybrid isoelectric focusing in an ongoing screening program for TTR variants in the Portuguese population with TTR Met 30 associated familial amyloidotic polyneuropathy. Cyanogen bromide peptide mapping and DNA restriction length polymorphism analyses showed that the propositus was a compound heterozygote for two TTR variants: Asn 90 and Met 119. Family analysis revealed that he inherited the TTR Met 119 variant from the mother and the TTR Asn 90 variant from the father. Neither the compound heterozygote nor his parents had symptoms of familial amyloidotic polyneuropathy. Serum dialysis with stepwise saturation of iodothyronine binding sites confirmed that TTR binding of T4 is increased in TTR Met 119. The increased binding is due to a higher TTR concentration rather than an increased association constant for T4. Because of the small proportion of serum T4 bound by TTR, increased T4 binding by TTR did not affect the ratio of free to bound T4 or T4 concentrations. In contrast, plasma retinol binding protein, almost all of which is bound by TTR, was elevated. The Asn 90 mutation does not affect either the concentration or the hormone binding characteristics of the protein. Possible long-term effects of these mutations and the combined heterozygotic state remain to be determined.

Thyroxine interactions with transthyretin: a comparison of 10 different naturally occurring human transthyretin variants.

Transthyretin (TTR) is a tetrameric protein that transports 15-20% of circulating T4. Alterations in TTR structure can manifest clinically as familial amyloidotic polyneuropathy or euthyroid hyperthyroxinemia. We have measured the relative affinity for T4 of several variant TTR molecules in human plasma. We have compared control plasma to plasma from a patient heterozygous for a [Thr109]TTR mutation associated with a 3-fold increased affinity for T4 and to plasma from patients with familial amyloidotic polyneuropathy with different point mutations in TTR. Relative affinity was measured in an assay in which [125I]T4 bound by TTR was isolated by immunoprecipitation with an antibody specific for TTR. [Thr109]TTR displayed the highest affinity for T4, whereas homozygous [Met30]TTR did not bind appreciable amounts of [125I]T4. The rank order of affinity of the various TTR mutations for T4 was: Thr109 heterozygous > wild type = Ala60 heterozygous = Ile122 heterozygous > His58 heterozygous approximately Tyr77 heterozygous approximately Ser84 heterozygous approximately Ile122 homozygous approximately Met30 heterozygous >> Met30 homozygous TTR. Thus, different point mutations in TTR increase, decrease, or do not affect TTR's affinity for T4. The ability of TTR to form amyloid fibrils does not appear to be related to its affinity for T4. Further study is required to define the molecular basis of alterations in T4 binding induced by point mutations located along the TTR tetramer.

Studies on the nature of iodothyronine binding in familial dysalbuminemic hyperthyroxinemia.

The effects of pH and anionic binding inhibitors were used to test the hypothesis that the increased T4 binding affinity of the variant albumin (Alb-FDH) of familial dysalbuminemic hyperthyroxinemia (FDH) is due to an electrostatic bond with the ionized phenolic hydroxyl of the iodothyronine. As determined by charcoal adsorption from 2% serum in which binding to T4-binding globulin and transthyretin had been inhibited, increased T4 binding by Alb-FDH was pH dependent and proportional to the ionization of the phenolic hydroxyl. Increased T3 binding became apparent above physiological pH, as is consistent with the higher pK of the T3 phenolic hydroxyl. The iodothyroacetic analogs of T4 and T3 developed maximal increases in binding to Alb-FDH at about the same pH as the corresponding iodothyronines. Aspirin, salicylate, warfarin, and chloride, anions that have minimal stereochemical resemblance to the iodothyronines but bind to albumin cationic groups, inhibited T4 binding to FDH sera at concentrations that had little or no effect on binding in normal sera. Increased displacement of T4 from Alb-FDH by salicylate was also evident at therapeutic ratios to a 1:1 dilution of serum in a dialysis system. Aspirin displaced T4 at a lower pH than T3, as is consistent with competition with the ionized iodothyronine phenolic group. These findings suggest that an electrostatic bond between the iodothyronine phenolate and a cationic group on the protein is the basis for the increased affinity and specificity of Alb-FDH for T4.

Artifactually elevated serum-free thyroxine levels measured by equilibrium dialysis in a pregnant woman with familial dysalbuminemic hyperthyroxinemia.

Familial dysalbuminemic hyperthyroxinemia (FDH) is a familial autosomal dominant syndrome caused by abnormal albumin with an increased affinity for thyroxine (T4). Two types of mutations in the albumin gene, replacing the normal arginine 218 with a histidine (R218H) or a proline (R218P), have been reported to cause FDH. Here, we report a pregnant Japanese woman with FDH caused by the mutant albumin R218P. She had extremely elevated total T4 levels but normal TSH. While the majority of T4was bound to albumin, T4 binding to thyroxine-binding globulin (TBG) was progressively increased throughout pregnancy. Her infant also had elevated serum T4 but normal thyrotropin (TSH). The presence of a guanine to cytosine transition in the second nucleotide of codon 218 of the albumin gene, resulting in a substitution of proline for the normal arginine (R218P), was revealed in the proband. Serum free thyroxine (FT4) levels were increased when measured with some commercial kits including equilibrium dialysis followed by radioimmunoassay (RIA) but not when determined by RIA after ultrafiltration of sera. These results indicate an increased T4 binding to TBG during pregnancy in the patients with FDH. Furthermore, our results suggest that normal serum FT4 determined by equilibrium dialysis is not an ultimate standard for the diagnosis of FDH in the patients with the mutant albumin R218P.

Transient prealbumin-associated hyperthyroxinemia in TSH-producing pituitary adenoma.

This case report describes a 38-year-old male who was hospitalized for further clarification of clinically mild hyperthyroidism. His increased total hormone levels, the elevated free thyroid hormones and the elevated basal TSH with blunted response to TRH strongly suggested a pituitary adenoma with inappropriate TSH incretion. Transmission computed tomography showed an intrasellar expansion, 16 mm in diameter. The neoplastic TSH production was confirmed by an elevated alpha-subunit and a raised molar alpha-sub/TSH ratio. However, T4 distribution on prealbumin (PA, TTR), albumin (A) and thyroxine binding globulin (TBG) showed a clearly increased binding to PA (39%), indicating additional prealbumin-associated hyperthyroxinemia. The absolute values of PA, A and TBG were within the normal range. After removal of the TSH-producing adenoma, basal TSH, the free thyroid hormones and T4 binding to prealbumin returned to normal. Therefore, the prealbumin-associated hyperthyroxinemia had to be interpreted as a transitory phenomenon related to secondary hyperthyroidism (T4 shift from thyroxine binding globulin to prealbumin) rather than a genetically conditioned anomaly of protein binding.

Elevated thyroxine levels in a euthyroid patient. A search for the cause of euthyroid hyperthyroxinemia.

A clinically euthyroid man with a family history of hyperthyroidism presented for evaluation of an elevated thyroxine (T4) level and an increased free T4 index with a normal thyrotropin (TSH) level. Results of thyroid hormone-binding protein tests confirmed the diagnosis of familial dysalbuminemic hyperthyroxinemia. This disorder should be considered in patients who have a normal serum TSH level, despite an elevated total T4 concentration. Accurate diagnosis is essential to avoid inappropriate treatment. Affected family members also should be identified. No treatment is required, because patients remain euthyroid and maintain a normal free T4 level.

[The effect of antioxidants on lipid metabolism in myocardium in experimental hyperthyroxinemic conditions]. / Wplyw antyutleniaczy na metabolizm lipidowy miesnia sercowego w warunkach doswiadczalnej hipertyroksynemii.

This experiment was conducted on 38 white Wistar-strain rats subdivided into five experimental groups. Group I were control animals which receiving intraperitoneal 0.5 ml injections of the sodium chloride physiological solution everyday during the whole experimental period of 26 days. Rats from Group II were injected intraperitoneally L-thyroxine (600 micrograms per kg of body weight) five times a week. The animals from Groups III, IV, and V, in addition to thyroxine injected in the same way as in Group II, were additionally administered one of such antioxidants as allopurinol, desferal, or vitamin C. The purpose of this research was to find an answer to the question whether the application of the above-mentioned antioxidants modifies the non-beneficial hyperthyroxinemic impact on the lipid metabolism of the cardiac muscle. It was found that the thyroxine influenced the change of the triglyceride and phospholipid contents in the cardiac muscle. All the applied antioxidants partly modified the thyroxine influence on the lipid balance in the cardiac muscle, especially in the area of triglycerides.

T4 plus T3 treatment in children with hypothyroidism and inappropriately elevated thyroid-stimulating hormone despite euthyroidism on T4 treatment.

AIMS: To evaluate the effect of addition of T3 to L-T4 treatment in children with congenital hypothyroidism (CH) who have inappropriately elevated thyroid-stimulating hormone (TSH) levels despite high normal serum T4 levels on L-T4 treatment. METHODS: Ten children (age 7.1 +/- 2 years) with CH whose TSH levels were persistently high despite euthyroidism and can only be normalized with hyperthyroidism were included. L-T4 treatment was switched to T3+L-T4 combination (Bitiron(R) tablet 50 mug L-T4 + 12.5 mug triiodothyronine). The patients received 50% of their usual L-T4 dose as L-T4 and the remaining half as T3 in a 4:1 ratio. The dose of T3+L-T4 was titrated to achieve normal TSH levels. Thyroid hormones and biochemical markers were followed for 1 year. RESULTS: Euthyrotropinemia was achieved at the 7th month (mean) of combination (T3+L-T4) treatment. Serum T4 and fT4 were lower and T3 was higher during combination compared to L-T4 treatment. LDL-cholesterol decreased and ALP increased in the euthyrotropinemic state. Vital signs were similar at hyperthyrotropinemia and euthyrotropinemia. CONCLUSION: T3+L-T4 treatment provides euthyrotropinemia without causing hyperthyroidism in children with CH and inappropriate hyperthyrotropinemia. Our data strongly suggest that decreased negative feedback due to lower T3 levels at the pituitary level is the main reason for persistent hyperthyrotropinemia.

Structural basis of albumin-thyroxine interactions and familial dysalbuminemic hyperthyroxinemia.

Human serum albumin (HSA) is the major protein component of blood plasma and serves as a transporter for thyroxine and other hydrophobic compounds such as fatty acids and bilirubin. We report here a structural characterization of HSA-thyroxine interactions. Using crystallographic analyses we have identified four binding sites for thyroxine on HSA distributed in subdomains IIA, IIIA, and IIIB. Mutation of residue R218 within subdomain IIA greatly enhances the affinity for thyroxine and causes the elevated serum thyroxine levels associated with familial dysalbuminemic hyperthyroxinemia (FDH). Structural analysis of two FDH mutants of HSA (R218H and R218P) shows that this effect arises because substitution of R218, which contacts the hormone bound in subdomain IIA, produces localized conformational changes to relax steric restrictions on thyroxine binding at this site. We have also found that, although fatty acid binding competes with thyroxine at all four sites, it induces conformational changes that create a fifth hormone-binding site in the cleft between domains I and III, at least 9 A from R218. These structural observations are consistent with binding data showing that HSA retains a high-affinity site for thyroxine in the presence of excess fatty acid that is insensitive to FDH mutations.

Mutations in a specific human serum albumin thyroxine binding site define the structural basis of familial dysalbuminemic hyperthyroxinemia.

The familial dysalbuminemic hyperthyroxinemia (FDH) phenotype results from a natural human serum albumin (HSA) mutant with histidine instead of arginine at amino acid position 218. This mutation results in an enhanced affinity for thyroxine. Site-directed mutagenesis and a yeast protein expression system were used to synthesize wild type HSA and FDH HSA as well as several other HSA mutants. Studies on the binding of thyroxine to these HSA species using equilibrium dialysis and quenching of tryptophan 214 fluorescence suggest that the FDH mutation affects a single thyroxine binding site located in the 2A subdomain of HSA. Site-directed mutagenesis of HSA and thyroxine analogs were used to obtain information about the mechanism of thyroxine binding to both wild type and FDH HSA. These studies suggest that the guanidino group of arginine at amino acid position 218 in wild type HSA is involved in an unfavorable binding interaction with the amino group of thyroxine, whereas histidine at amino acid position 218 in FDH HSA is involved in a favorable binding interaction with thyroxine. Neither arginine at amino acid position 222 nor tryptophan at amino acid position 214 appears to favorably influence the binding of thyroxine to wild type HSA.

Mutagenesis studies of thyroxine binding to human serum albumin define an important structural characteristic of subdomain 2A.

The familial dysalbuminemic hyperthyroxinemia (FDH) phenotype results from a natural human serum albumin (HSA) mutant, with histidine instead of arginine at amino acid position 218. This mutation results in an enhanced affinity for thyroxine. In our earlier study, site-directed mutagenesis and a yeast protein expression system were used to synthesize FDH HSA and several other HSA mutants. Measurement of the binding of these HSA mutants to thyroxine and several thyroxine analogs using equilibrium dialysis and quenching of tryptophan 214 fluorescence allowed us to propose a preliminary model of thyroxine binding to the 2A subdomain of wild type and FDH HSA. In this study, we have produced several other HSA mutants. By comparing the binding affinity of these mutants for thyroxine and tetraiodothyroacetic acid to the binding affinity of other mutants, we were able to suggest a new model for thyroxine binding to the 2A subdomain of HSA. We found that the substitution of arginine at position 218 with alanine increased the binding affinity for thyroxine by 2 orders of magnitude relative to the binding affinity of wild type HSA for thyroxine. A more accurate understanding of the mechanism of thyroxine binding to HSA has allowed us to define an important structural characteristic of subdomain 2A, one of the two principal binding sites on HSA for small hydrophobic ligands.