Consensus on criteria for acromegaly diagnosis and remission.

PURPOSE: The 14th Acromegaly Consensus Conference was convened to consider biochemical criteria for acromegaly diagnosis and evaluation of therapeutic efficacy. METHODS: Fifty-six acromegaly experts from 16 countries reviewed and discussed current evidence focused on biochemical assays; criteria for diagnosis and the role of imaging, pathology, and clinical assessments; consequences of diagnostic delay; criteria for remission and recommendations for follow up; and the value of assessment and monitoring in defining disease progression, selecting appropriate treatments, and maximizing patient outcomes. RESULTS: In a patient with typical acromegaly features, insulin-like growth factor (IGF)-I > 1.3 times the upper limit of normal for age confirms the diagnosis. Random growth hormone (GH) measured after overnight fasting may be useful for informing prognosis, but is not required for diagnosis. For patients with equivocal results, IGF-I measurements using the same validated assay can be repeated, and oral glucose tolerance testing might also be useful. Although biochemical remission is the primary assessment of treatment outcome, biochemical findings should be interpreted within the clinical context of acromegaly. Follow up assessments should consider biochemical evaluation of treatment effectiveness, imaging studies evaluating residual/recurrent adenoma mass, and clinical signs and symptoms of acromegaly, its complications, and comorbidities. Referral to a multidisciplinary pituitary center should be considered for patients with equivocal biochemical, pathology, or imaging findings at diagnosis, and for patients insufficiently responsive to standard treatment approaches. CONCLUSION: Consensus recommendations highlight new understandings of disordered GH and IGF-I in patients with acromegaly and the importance of expert management for this rare disease.

Does testosterone influence the association between sleep and frailty in men: results from the European Male Aging Study.

BACKGROUND: Previous studies have suggested an association between sleep disturbance and frailty. The mechanism is unknown, although it has been suggested that hormonal factors may play a role. METHODS: The aim was to determine the association between sleep duration, sleep quality and frailty, and to determine whether testosterone influenced this association. Males aged 40-79 years were recruited from eight European centres to the European Male Aging Study (EMAS). Subjects completed an interviewer-assisted questionnaire including questions regarding sleep quality and duration. Sleep quality was scored 0-20 and categorised as 0-4, 5-9, 10-14, and 15-20, with higher scores indicating poorer quality. A 39-component frailty index (FI) was constructed. Total testosterone levels were measured. The association between sleep duration, sleep quality and the FI was assessed using negative binomial regression, with adjustment for putative confounders including testosterone level. RESULTS: Two thousand three hundred ninety-three participants contributed data to the analysis. The mean age was 63.3 years and mean sleep duration was 7.01 h. The mean frailty index was 0.15. Mean testosterone levels declined with decreasing sleep quality. After adjustment, compared to those with a sleep score of 0-4, the FI was 57% (95% CI 38%, 78%) higher among those with a sleep score of 15-20. After adjustment compared to those with normal sleep duration (6-9 h), those with a short (< 6 h) and long (≥ 9 h) sleep duration had a 16% (95% CI 6%, 28%) and 11% (95% CI 0%, 23%) higher FI, respectively. Adjustment for testosterone did not influence the strength of either association. CONCLUSION: Frailty is associated with impaired sleep quality and sleep duration. The association cannot, however, be explained by variation in testosterone levels.

Natural history and comorbidities of generalised and partial lipodystrophy syndromes in Spain.

The rarity of lipodystrophies implies that they are not well-known, leading to delays in diagnosis/misdiagnosis. The aim of this study was to assess the natural course and comorbidities of generalised and partial lipodystrophy in Spain to contribute to their understanding. Thus, a total of 140 patients were evaluated (77.1% with partial lipodystrophy and 22.9% with generalised lipodystrophy). Clinical data were collected in a longitudinal setting with a median follow-up of 4.7 (0.5-17.6) years. Anthropometry and body composition studies were carried out and analytical parameters were also recorded. The estimated prevalence of all lipodystrophies in Spain, excluding Köbberling syndrome, was 2.78 cases/million. The onset of phenotype occurred during childhood in generalised lipodystrophy and during adolescence-adulthood in partial lipodystrophy, with the delay in diagnosis being considerable for both cohorts. There are specific clinical findings that should be highlighted as useful features to take into account when making the differential diagnosis of these disorders. Patients with generalised lipodystrophy were found to develop their first metabolic abnormalities sooner and a different lipid profile has also been observed. Mean time to death was 83.8 ± 2.5 years, being shorter among patients with generalised lipodystrophy. These results provide an initial point of comparison for ongoing prospective studies such as the ECLip Registry study.

Vitamin D status and supplementation before and after Bariatric Surgery: Recommendations based on a systematic review and meta-analysis.

Bariatric surgery is associated with a postoperative reduction of 25(OH) vitamin D levels (25(OH)D) and with skeletal complications. Currently, guidelines for 25(OH)D assessment and vitamin D supplementation in bariatric patients, pre- and post-surgery, are still lacking. The aim of this work is to analyse systematically the published experience on 25(OH)D status and vitamin D supplementation, pre- and post-surgery, and to propose, on this basis, recommendations for management. Preoperatively, 18 studies including 2,869 patients were evaluated. Prevalence of vitamin D insufficiency as defined by 25(OH)D < 30 ng/mL (75 nmol/L) was 85%, whereas when defined by 25(OH)D < 20 ng/mL (50 nmol/L) was 57%. The median preoperative 25(OH)D level was 19.75 ng/mL. After surgery, 39 studies including 5,296 patients were analysed and among those undergoing either malabsorptive or restrictive procedures, a lower rate of vitamin D insufficiency and higher 25(OH)D levels postoperatively were observed in patients treated with high-dose oral vitamin D supplementation, defined as ≥ 2,000 IU/daily (mostly D3-formulation), compared with low-doses (< 2,000 IU/daily). Our recommendations based on this systematic review and meta-analysis should help clinical practice in the assessment and management of vitamin D status before and after bariatric surgery. Assessment of vitamin D should be performed pre- and postoperatively in all patients undergoing bariatric surgery. Regardless of the type of procedure, high-dose supplementation is recommended in patients after bariatric surgery.

Diagnosis and management of prolactin-secreting pituitary adenomas: a Pituitary Society international Consensus Statement.

This Consensus Statement from an international, multidisciplinary workshop sponsored by the Pituitary Society offers evidence-based graded consensus recommendations and key summary points for clinical practice on the diagnosis and management of prolactinomas. Epidemiology and pathogenesis, clinical presentation of disordered pituitary hormone secretion, assessment of hyperprolactinaemia and biochemical evaluation, optimal use of imaging strategies and disease-related complications are addressed. In-depth discussions present the latest evidence on treatment of prolactinoma, including efficacy, adverse effects and options for withdrawal of dopamine agonist therapy, as well as indications for surgery, preoperative medical therapy and radiation therapy. Management of prolactinoma in special situations is discussed, including cystic lesions, mixed growth hormone-secreting and prolactin-secreting adenomas and giant and aggressive prolactinomas. Furthermore, considerations for pregnancy and fertility are outlined, as well as management of prolactinomas in children and adolescents, patients with an underlying psychiatric disorder, postmenopausal women, transgender individuals and patients with chronic kidney disease. The workshop concluded that, although treatment resistance is rare, there is a need for additional therapeutic options to address clinical challenges in treating these patients and a need to facilitate international registries to enable risk stratification and optimization of therapeutic strategies.

Number of CAG repeats and mortality in middle aged and older men.

DESIGN: The androgen receptor (AR) mediates peripheral effects of testosterone. Previous data suggests an association between the number of CAG repeats in exon-1 of the AR gene and AR transcriptional activity. The aim of this analysis was to determine the association between the number of AR CAG repeats and all-cause mortality in men and the influence of testosterone level on the association. PATIENTS AND MEASUREMENTS: Follow-up data to 27 January 2018 were available for men aged 40-79 years recruited across six countries of the European Male Aging Study between 2003 and 2005. Cox proportional hazards modelling was used to determine the association between CAG repeat number/mortality. Results were expressed as hazard ratios (HR)/95% confidence intervals (CI). RESULTS: One thousand nine hundred and seventy-seven men were followed up. Mean baseline age was 60 ± 11.1 years. Mean duration of follow-up was 12.2 years. At follow up 25.1% of men had died. CAG repeat length ranged from 6 to 39, with the highest proportion of CAG repeat number at 21 repeats (16.4%). In a multivariable model, compared to men with 22-23 AR CAG repeats: for men with <22 and >23 AR CAG HR, 95% CI for mortality were, <22 CAG repeats 1.17 (0.93-1.49) and >23 CAG repeats 1.14 (0.88-1.47). In a post-hoc analysis, the association was significant for men in the lowest tertile of baseline testosterone (<14.2 nmol/L) with >23 CAG repeats: in the adjusted model for <22 and >23 CAG repeats, respectively, 1.49 (0.97-2.27) and 1.68 (1.06-2.67) versus 22-23 repeats. CONCLUSIONS: Our European-wide cohort data overall found no association of androgen receptor CAG repeat number and mortality in men. However, post hoc analysis suggested that an association might be present in men with lower baseline testosterone concentrations, which merits further investigation.

GLP-1 Increases Circulating Leptin Levels in Truncal Vagotomized Rats.

GLP-1 is a gastro-intestinal hormone acting within the gut/brain axis for energy balance regulation. We aimed to evaluate the role of the vagus nerve in whole-body energy homeostasis and in mediating GLP-1 effects. For this, rats submitted to truncal vagotomy and sham-operated controls underwent a comprehensive evaluation, including eating behavior, body weight, percentage of white (WAT) and brown adipose tissue (BAT), resting energy expenditure (REE) and acute response to GLP-1. Truncal vagotomized rats had significantly lower food intake, body weight, body weight gain, WAT and BAT, with a higher BAT/WAT ratio, but no significant difference in REE when compared to controls. Vagotomized rats also had significantly higher fasting ghrelin and lower glucose and insulin levels. After GLP-1 administration, vagotomized rats depicted a blunted anorexigenic response and higher plasma leptin levels, as compared to controls. However, in vitro stimulation of VAT explants with GLP-1 resulted in no significant changes in leptin secretion. In conclusion, the vagus nerve influences whole-body energy homeostasis by modifying food intake, body weight and body composition and by mediating the GLP-1 anorectic response. The higher leptin levels in response to acute GLP-1 administration observed after truncal vagotomy suggest the existence of a putative GLP-1-leptin axis that relies on the integrity of gut-brain vagal pathway.

Sarcopenia-defining parameters, but not sarcopenia, are associated with cognitive domains in middle-aged and older European men.

BACKGROUND: Previous research suggests that sarcopenia is associated with lower cognitive functioning. Evidence on the longitudinal relationship between cognition and sarcopenia, according to the revised criteria of the European Working Group on Sarcopenia in Older People (EWGSOP2), is scarce. This study aimed to investigate both cross-sectional and longitudinal associations between sarcopenia and its defining parameters (muscle strength, muscle mass and physical performance) and cognitive performance in middle-aged and older men. METHODS: This was a secondary analysis of data from the European Male Ageing Study (EMAS), a multicentre cohort study of men aged 40-79 years, recruited from population registers in eight European centres. Cognitive functioning was assessed by using a battery of three neuropsychological tests, measuring fluid intelligence: Rey-Osterrieth Complex Figure (ROCF-Copy and ROCF-Recall), Camden Topographical Recognition Memory (CTRM) and Digit Symbol Substitution Test (DSST). Sarcopenia-defining parameters appendicular lean mass (aLM), gait speed (GS), chair stand test (CST) and handgrip strength (HGS) were measured. Sarcopenia was diagnosed according to the criteria of the EWGSOP2. All measurements were performed at baseline and after a follow-up of 4.3 years. Cross-sectional associations between cognition, sarcopenia-defining parameters and prevalent sarcopenia (EWGSOP2) were analysed. Longitudinally, the predictive value of baseline cognition on decline in sarcopenia-defining parameters, onset of new sarcopenia and vice versa was examined. Linear and logistic regression were used and adjusted for putative confounders. RESULTS: In the whole cohort (n = 3233), ROCF-Copy (ß = 0.016; P < 0.05), ROCF-Recall (ß = 0.010; P < 0.05), CTRM (ß = 0.015; P < 0.05), DSST score (ß = 0.032; P < 0.05) and fluid cognition (ß = 0.036; P < 0.05) were significantly and independently associated with GS at baseline. In the Leuven + Manchester subcohorts (n = 456), ROCF-Copy (ß = 1.008; P < 0.05), ROCF-Recall (ß = 0.908; P < 0.05) and fluid cognition (ß = 1.482; P < 0.05) were associated with HGS. ROCF-Copy (ß = 0.394; P < 0.05), ROCF-Recall (ß = 0.316; P < 0.05), DSST (ß = 0.393; P < 0.05) and fluid cognition (ß = 0.765; P < 0.05) were associated with aLM. The prevalence of sarcopenia in this population was 17.8%. No associations were detected between cognition and prevalent or incident sarcopenia. Longitudinal analysis showed that low ROCF-Copy score at baseline was associated with an increase in CST in men ≥70 years (ß = -0.599; P < 0.05). In addition, a decrease in ROCF-Recall was associated with a decrease in GS, and a decrease in DSST was associated with an increase in CST (ß = 0.155; P < 0.0001, ß = -0.595; P < 0.001, respectively) in persons with the highest change in both cognition and muscle function. CONCLUSIONS: Sarcopenia was not associated with cognitive performance in this population, whereas several components of sarcopenia were associated with domain-specific cognitive performance. Longitudinally, baseline and change in subdomains of cognition predicted change in muscle function in specific subgroups.

Gender Dimorphism in Hepatic Carcinogenesis-Related Gene Expression Associated with Obesity as a Low-Grade Chronic Inflammatory Disease.

Non-alcoholic fatty liver disease (NAFLD) and hepatocellular carcinoma (HCC) show clear evidence of sexual dimorphism, with a significantly higher incidence in males. Among the determining factors that could explain this sex-based difference, the specific distribution of fat by sex has been suggested as a primary candidate, since obesity is a relevant risk factor. In this context, obesity, considered a low-grade chronic inflammatory pathology and responsible for the promotion of liver disease, could lead to sexual dimorphism in the expression profile of genes related to tumor development. When we compared the expression levels of genes associated with the early stages of carcinogenesis in the liver between male and female diet-induced obesity (DIO) rats, we observed that the expression pattern was similar in obese male and female animals. Interestingly, the SURVIVIN/BIRC5 oncogene showed a higher expression in male DIO rats than in female DIO and lean rats. This trend related to sexual dimorphism was observed in leukocytes from patients with obesity, although the difference was not statistically significant. In conclusion, this study evidenced a similar pattern in the expression of most carcinogenesis-related genes in the liver, except SUVIVIN/BIRC5, which could be a predictive biomarker of liver carcinogenesis predisposition in male patients with obesity.

The Leydig cell biomarker INSL3 as a predictor of age-related morbidity: Findings from the EMAS cohort.

Background: Insulin-like peptide 3 (INSL3) is a constitutive hormone secreted in men by the mature Leydig cells of the testes. It is an accurate biomarker for Leydig cell functional capacity, reflecting their total cell number and differentiation status. Objectives: To determine the ability of INSL3 to predict hypogonadism and age-related morbidity using the EMAS cohort of older community-dwelling men. Materials & methods: Circulating INSL3 was assessed in the EMAS cohort and its cross-sectional and longitudinal relationships to hypogonadism, here defined by testosterone (T) <10.5nmol/l, and a range of age-related morbidities determined by correlation and regression analysis. Results & discussion: While INSL3 is an accurate measure of primary hypogonadism, secondary and compensated hypogonadism also indicate reduced levels of INSL3, implying that testicular hypogonadism does not improve even when LH levels are increased, and that ageing-related hypogonadism may combine both primary and secondary features. Unadjusted, serum INSL3, like calculated free testosterone (cFT), LH, or the T/LH ratio reflects hypogonadal status and is associated with reduced sexual function, bone mineral density, and physical activity, as well as increased occurrence of hypertension, cardiovascular disease, cancer, and diabetes. Using multiple regression analysis to adjust for a range of hormonal, anthropometric, and lifestyle factors, this relationship is lost for all morbidities, except for reduced bone mineral density, implying that INSL3 and/or its specific receptor, RXFP2, may be causally involved in promoting healthy bone metabolism. Elevated INSL3 also associates with hypertension and cardiovascular disease. When unadjusted, INSL3 in phase 1 of the EMAS study was assessed for its association with morbidity in phase 2 (mean 4.3 years later); INSL3 significantly predicts 7 out of 9 morbidity categories, behaving as well as cFT in this regard. In contrast, total T was predictive in only 3 of the 9 categories. Conclusion: Together with its low within-individual variance, these findings suggest that assessing INSL3 in men could offer important insight into the later development of disease in the elderly.

Brown Adipose Tissue Sheds Extracellular Vesicles That Carry Potential Biomarkers of Metabolic and Thermogenesis Activity Which Are Affected by High Fat Diet Intervention.

Brown adipose tissue (BAT) is a key target for the development of new therapies against obesity due to its role in promoting energy expenditure; BAT secretory capacity is emerging as an important contributor to systemic effects, in which BAT extracellular vesicles (EVs) (i.e., batosomes) might be protagonists. EVs have emerged as a relevant cellular communication system and carriers of disease biomarkers. Therefore, characterization of the protein cargo of batosomes might reveal their potential as biomarkers of the metabolic activity of BAT. In this study, we are the first to isolate batosomes from lean and obese Sprague-Dawley rats, and to establish reference proteome maps. An LC-SWATH/MS analysis was also performed for comparisons with EVs secreted by white adipose tissue (subcutaneous and visceral WAT), and it showed that 60% of proteins were exclusive to BAT EVs. Precisely, batosomes of lean animals contain proteins associated with mitochondria, lipid metabolism, the electron transport chain, and the beta-oxidation pathway, and their protein cargo profile is dramatically affected by high fat diet (HFD) intervention. Thus, in obesity, batosomes are enriched with proteins involved in signal transduction, cell communication, the immune response, inflammation, thermogenesis, and potential obesity biomarkers including UCP1, Glut1, MIF, and ceruloplasmin. In conclusion, the protein cargo of BAT EVs is affected by the metabolic status and contains potential biomarkers of thermogenesis activity.

Prolonged breastfeeding protects from obesity by hypothalamic action of hepatic FGF21.

Early-life determinants are thought to be a major factor in the rapid increase of obesity. However, while maternal nutrition has been extensively studied, the effects of breastfeeding by the infant on the reprogramming of energy balance in childhood and throughout adulthood remain largely unknown. Here we show that delayed weaning in rat pups protects them against diet-induced obesity in adulthood, through enhanced brown adipose tissue thermogenesis and energy expenditure. In-depth metabolic phenotyping in this rat model as well as in transgenic mice reveals that the effects of prolonged suckling are mediated by increased hepatic fibroblast growth factor 21 (FGF21) production and tanycyte-controlled access to the hypothalamus in adulthood. Specifically, FGF21 activates GABA-containing neurons expressing dopamine receptor 2 in the lateral hypothalamic area and zona incerta. Prolonged breastfeeding thus constitutes a protective mechanism against obesity by affecting long-lasting physiological changes in liver-to-hypothalamus communication and hypothalamic metabolic regulation.

Novel approaches to bone comorbidity in Cushing's disease: an update.

Skeletal complications are frequent and clinically relevant findings in Cushing's disease (CD) since an uncoupled suppressed bone formation and enhanced bone resorption leads to a marked skeletal damage with a rapid increase of fracture risk. Reduced Bone Mineral Density (BMD) has been consistently reported and osteopenia or osteoporosis are typical findings in patients with CD. Vertebral Fractures (VFs) are frequently reported and may occur even in patients with an only mild reduction of BMD, representing nowadays a still under- or misdiagnosed comorbidity of these patients being frequently asymptomatic. A novel approach combining different available tools such as BMD evaluation and vertebral morphometry, in order to improve diagnosis, management, and follow-up of bone comorbidity in all patients affected by CD, is needed. This approach is foreseen to be a crucial part of management of patients with CD, particularly in Pituitary Tumor Center of Excellence since VFs, the landmark of the bone involvement, may occur early in the history of the disease and may represent a relevant risk factor for further fractures, reduced quality of life and survival and need for pharmacologic prevention and treatment.

Genetic Variants in Folate and Cobalamin Metabolism-Related Genes in Pregnant Women of a Homogeneous Spanish Population: The Need for Revisiting the Current Vitamin Supplementation Strategies.

Polymorphisms of genes involved in the metabolism and transport of folate and cobalamin could play relevant roles in pregnancy outcomes. This study assessed the prevalence of genetic polymorphisms of folate and cobalamin metabolism-related genes such as MTHFR, MTR, CUBN, and SLC19A1 in pregnant women of a homogeneous Spanish population according to conception, pregnancy, delivery, and newborns complications. This study was conducted on 149 nulliparous women with singleton pregnancies. Sociodemographic and obstetrics variables were recorded, and all patients were genotyped in the MTHFR, MTR, CUBN, and SLC10A1 polymorphisms. The distribution of genotypes detected in this cohort was similar to the population distribution reported in Europe, highlighting that more than 50% of women were carriers of risk alleles of the studied genes. In women with the MTHFR risk allele, there was a statistically significant higher frequency of assisted fertilisation and a higher frequency of preeclampsia and preterm birth. Moreover, CUBN (rs1801222) polymorphism carriers showed a statistically significantly lower frequency of complications during delivery. In conclusion, the prevalence of genetic variants related to folic acid and vitamin B12 metabolic genes in pregnant women is related to mother and neonatal outcomes. Knowing the prevalence of these polymorphisms may lead to a personalised prescription of vitamin intake.

Immunomodulatory effect of a very-low-calorie ketogenic diet compared with bariatric surgery and a low-calorie diet in patients with excessive body weight.

BACKGROUND & AIM: Inflammation and oxidative stress are the most probable mechanistic link between obesity and its co-diseases with cancer among them. The aim of this study was to evaluate whether the nutritional ketosis and weight loss induced by a very-low-calorie ketogenic diet (VLCKD) modulates the inflammatory and oxidative stress profile, compared with a standard, balanced hypocaloric diet (LCD) or bariatric surgery (BS) in patients with obesity. METHODS: The study was performed in 79 patients with overweight or obesity and 32 normal-weight volunteers as the control group. Patients with obesity underwent a weight reduction therapy based on VLCKD, LCD or BS. The quantification of the circulating levels of a multiplexing test of cytokines and carcinogenesis/aging biomarkers, as well as of lipid peroxides and total antioxidant power, was carried out. RESULTS: First, we observed that pro-inflammatory cytokines increase, while anti-inflammatory cytokines decrease under excessive body weight. Relevantly, when patients underwent weight loss strategies, it was shown that energy-restricted and surgical strategies of weight loss induced changes in circulating cytokine and lipid peroxides. This effect was more notable in patients following the VLCKD than the LCD or BS and it was observed mainly in the ketosis phase of the intervention. Particularly, IL-11, IL-12, IL-2, INF-γ, INF-ß, Pentraxin-3 or MMP1 changed after VLCKD. Whereas, APRIL, TWEAK, osteocalcin and IL-28A increased after BS. CONCLUSION: As far as we know, this is the first study that evaluate the time-course of cytokines and oxidative stress markers after a VLCKD as compared with a standard LCD and BS. The observed results support the immunomodulatory effect of nutritional ketosis induced by a VLCKD synergistically with weight loss as a strategy to improve innate-immunity and to prevent infections and carcinogenesis in patients with obesity.

Association of age, hormonal, and lifestyle factors with the Leydig cell biomarker INSL3 in aging men from the European Male Aging Study cohort.

BACKGROUND: Aging in men is accompanied by a broad range of symptoms, including sexual dysfunction, cognitive and musculoskeletal decline, obesity, type 2 diabetes, cardiovascular disease and hypertension, organ degeneration/failure, and increasing neoplasia, some of which are associated with declining levels of Leydig cell-produced testosterone. High natural biological variance, together with multiple factors that can modulate circulating testosterone concentration, may influence its interpretation and clinical implications. Insulin-like peptide 3 is a biomarker of Leydig cell function that might provide complementary information on testicular health and its downstream outcomes. OBJECTIVES: To characterize insulin-like peptide 3 as a biomarker to assess gonadal status in aging men. METHODS AND MATERIALS: A large European multicenter (European Male Aging Study) cohort of community-dwelling men was analyzed to determine how insulin-like peptide 3 relates to a range of hormonal, anthropometric, and lifestyle parameters. RESULTS AND DISCUSSION: Insulin-like peptide 3 declines cross-sectionally and longitudinally within individuals at approximately 15% per decade from age 40 years, unlike testosterone (1.9% per decade), which is partly compensated by increasing pituitary luteinizing hormone production. Importantly, lower insulin-like peptide 3 in younger men appears to persist with aging. Multiple regression analysis shows that, unlike testosterone, insulin-like peptide 3 is negatively dependent on luteinizing hormone and sex hormone-binding globulin and positively dependent on follicle-stimulating hormone, suggesting a different mechanism of gonadotropic regulation. Circulating insulin-like peptide 3 is negatively associated with increased body mass index or waist circumference and with smoking, and unlike testosterone, it is not affected by weight loss in obese individuals. Geographic variation in mean insulin-like peptide 3 within Europe appears to be largely explained by differences in these parameters. The results allowed the establishment of a European-wide reference range for insulin-like peptide 3 (95% confidence interval) adjusted for increasing age. CONCLUSION: Insulin-like peptide 3 is a constitutive biomarker of Leydig cell functional capacity and is a robust, reliably measurable peptide not subject to gonadotropin-dependent short-term regulation and within-individual variation in testosterone.