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.

IGSF1 Does Not Regulate Spermatogenesis or Modify FSH Synthesis in Response to Inhibins or Activins.

Loss-of-function mutations in the X-linked immunoglobulin superfamily, member 1 (IGSF1) gene result in central hypothyroidism, often associated with macroorchidism. Testicular enlargement in these patients might be caused by increases in follicle-stimulating hormone (FSH) levels, as IGSF1 has been proposed to function as an inhibin B receptor or as an inhibitor of activin type I receptor (ALK4) activity in pituitary gonadotrope cells. If true, loss of IGSF1 should lead to reduced inhibin B action or disinhibition of activin signaling, thereby increasing FSH synthesis. Here, we show that FSH levels and sperm counts are normal in male Igsf1 knockout mice, although testis size is mildly increased. Sperm parameters are also normal in men with IGSF1 deficiency, although their FSH levels may trend higher and their testes are enlarged. Inhibin B retains the ability to suppress FSH synthesis in pituitaries of Igsf1-knockout mice and IGSF1 does not interact with ALK4 or alter activin A/ALK4 stimulation of FSHß (Fshb/FSHB) subunit transcription or expression. In light of these results, it is unlikely that macroorchidism in IGSF1 deficiency derives from alterations in spermatogenesis or inhibin/activin regulation of FSH.

Age-Specific Reference Intervals for Plasma Free Thyroxine and Thyrotropin in Term Neonates During the First Two Weeks of Life.

Background: Congenital hypothyroidism (CH) is a common and preventable cause of mental retardation, which is detected in many neonatal screening programs. Upon suspicion of CH, plasma free thyroxine (fT4) and thyrotropin (TSH) concentrations are measured. CH can be of thyroidal or central origin (CH-T and CH-C, respectively). While CH-T diagnosis is based on an elevated TSH with a low fT4, CH-C diagnosis is based on a low fT4 without a clearly elevated TSH. Currently, reliable neonatal reference intervals (RIs) for plasma fT4 and TSH are lacking. Age-specific RIs would greatly improve the diagnostic process for CH, especially for CH-C. Our aim was to establish neonatal RIs for plasma fT4 and TSH in term neonates at day 3-7 (t = 1) and day 13-15 (t = 2). The study was particularly designed to provide a reliable fT4 lower limit of the RI to facilitate the diagnosis of CH-C. In the Netherlands, neonates are screened at day 3-7 of life. After a screening result suggestive for CH-C, pediatric consultation takes place on average at day 14. Thus, the time points were chosen accordingly. Methods: Venous blood was collected from 120 healthy neonates at each time point (94 participants provided blood samples at two time points; 52 participants provided a sample at t = 1 or t = 2). fT4 and TSH were measured using an immunoassay (Cobas; Roche Diagnostics). RIs were calculated using the 95% confidence interval for normally distributed data and the nonparametric percentile method if data were not normally distributed. Results: From 146 participants (49% female), ≥1 measurement was available. Ninety-five percent RIs for fT4 were 20.5-37.1 pmol/L (day 3-7) and 15.3-26.5 pmol/L (day 13-15). Ninety-five percent RIs for TSH were 1.0-8.4 mU/L (day 3-7) and 1.4-8.6 mU/L (day 13-15). Conclusions: Our results indicate an fT4 lower limit of the RI of 20.5 pmol/L at day 3-7 and 15.3 pmol/L at day 13-15. These lower limits are considerably higher than this assay's lower limit of the adult RI for fT4. In case CH is suspected, we recommend measuring fT4 and TSH using an assay with an established neonatal RI, taking into account the child's age in days.

Mutations in IRS4 are associated with central hypothyroidism.

BACKGROUND: Four genetic causes of isolated congenital central hypothyroidism (CeH) have been identified, but many cases remain unexplained. We hypothesised the existence of other genetic causes of CeH with a Mendelian inheritance pattern. METHODS: We performed exome sequencing in two families with unexplained isolated CeH and subsequently Sanger sequenced unrelated idiopathic CeH cases. We performed clinical and biochemical characterisation of the probands and carriers identified by family screening. We investigated IRS4 mRNA expression in human hypothalamus and pituitary tissue, and measured serum thyroid hormones and Trh and Tshb mRNA expression in hypothalamus and pituitary tissue of Irs4 knockout mice. RESULTS: We found mutations in the insulin receptor substrate 4 (IRS4) gene in two pairs of brothers with CeH (one nonsense, one frameshift). Sequencing of IRS4 in 12 unrelated CeH cases negative for variants in known genes yielded three frameshift mutations (two novel) in three patients and one male sibling. All male carriers (n=8) had CeH with plasma free thyroxine concentrations below the reference interval. MRI of the hypothalamus and pituitary showed no structural abnormalities (n=12). 24-hour thyroid-stimulating hormone (TSH) secretion profiles in two adult male patients showed decreased basal, pulsatile and total TSH secretion. IRS4 mRNA was expressed in human hypothalamic nuclei, including the paraventricular nucleus, and in the pituitary gland. Female knockout mice showed decreased pituitary Tshb mRNA levels but had unchanged serum thyroid hormone concentrations. CONCLUSIONS: Mutations in IRS4 are associated with isolated CeH in male carriers. As IRS4 is involved in leptin signalling, the phenotype may be related to disrupted leptin signalling.

Administration of Thyrotropin-Releasing Hormone in the Hypothalamic Paraventricular Nucleus of Male Rats Mimics the Metabolic Cold Defense Response.

BACKGROUND: Cold exposure increases thyrotropin-releasing hormone (TRH) expression primarily in the hypothalamic paraventricular nucleus (PVN). The PVN is a well-known hypothalamic hub in the control of energy metabolism. TRH terminals and receptors are found on PVN neurons. We hypothesized that TRH release in the PVN plays an important role in the control of thermogenesis and energy mobilization during cold exposure. METHODS: Male Wistar rats were exposed to a cold environment (4°C) or TRH retrodialysis in the PVN for 2 h. We compared the effects of cold exposure and TRH administration in the PVN on plasma glucose, corticosterone, and thyroid hormone concentrations, body temperature, locomotor activity, as well as metabolic gene expression in the liver and brown adipose tissue. RESULTS: Cold exposure increased body temperature, locomotor activity, and plasma corticosterone concentrations, but blood glucose concentrations were similar to that of room temperature control animals. TRH administration in the PVN also promptly increased body temperature, locomotor activity and plasma corticosterone concentrations. However, TRH administration in the PVN markedly increased blood glucose concentrations and endogenous glucose production (EGP) compared to saline controls. Selective hepatic sympathetic or parasympathetic denervation reduced the TRH-induced increase in glucose concentrations and EGP. Gene expression data indicated increased gluconeogenesis in liver and lipolysis in brown adipose tissue, both after cold exposure and TRH administration. CONCLUSIONS: We conclude that TRH administration in the rat PVN largely mimics the metabolic and behavioral changes induced by cold exposure indicating a potential link between TRH release in the PVN and cold defense.

Effects of intravenous thyrotropin-releasing hormone on 18F-fluorodeoxyglucose uptake in human brown adipose tissue: a randomized controlled trial.

OBJECTIVE: Brown adipose tissue (BAT) activity in humans is stimulated by cold and by a limited number of pharmacological agents, including ß3-adrenergic agonists and bile acids. Although thyrotropin-releasing hormone (TRH) is known to activate BAT in several mammals, this has not been reported in humans. DESIGN: A randomized, placebo-controlled, double-blind, cross-over trial. METHODS: We investigated the effects of intravenous bolus administration of 400 µg TRH or 2 mL saline on BAT activity in healthy, lean men. BAT activity was measured as standardized 18F-fluorodeoxyglucose (18F-FDG) uptake and glucose metabolic rate (MRglu) using dynamic PET/CT imaging. The first six individuals were studied at room temperature, while subsequently nine were exposed to mild cold (17°C ± 1°C) for 60 min before imaging. During the dynamic scan, blood was withdrawn for measurement of thyroid hormone and catecholamine concentrations. This trial is registered with The Netherlands National Trial Register (number NTR5512). RESULTS: Sixteen participants were recruited. Six men studied at room temperature showed no visible BAT activity during either session. After exposure to mild cold, four of nine men (44.4%) showed clear increase of 18F-FDG uptake after TRH administration compared to placebo. Maximal standardized 18F-FDG uptake showed a trend toward increase after TRH compared to placebo (P = 0.066). MRglu showed a significant increase after TRH administration (P = 0.014). The increase in 18F-FDG uptake was not paralleled by changes in plasma thyroid hormone or catecholamine concentrations. CONCLUSION: Systemic TRH administration can increase the activity of cold-stimulated BAT in adult men. These findings may assist developing pharmacological strategies for modulating BAT activity in the management of obesity.

A Novel IGSF1 Mutation in a Boy With Short Stature and Hypercholesterolemia: A Case Report.

CONTEXT: IGSF1 deficiency is a recently discovered syndrome consisting of congenital central hypothyroidism (CeH) and macroorchidism. Here, we report on a patient presenting with short stature, who was found to carry a pathogenic mutation in the IGSF1 gene. CASE DESCRIPTION: A 14-year-old Israeli boy was referred to the Academic Medical Center in Amsterdam, The Netherlands, for follow-up on short stature ascribed to constitutional delay of growth and puberty, and familial hypercholesterolemia. Primary hypothyroidism had previously been excluded by a normal thyroid-stimulating hormone (TSH) concentration. However, in follow-up, plasma free thyroxine (FT4) concentrations were repeatedly low, and the patient was diagnosed with CeH. Because of coexistent relative macroorchidism, IGSF1 gene analysis was performed, revealing a mutation (c.2588C>G; p.Ser863Cys). The mutant IGSF1 protein was retained mainly in the endoplasmic reticulum and reached the plasma membrane with poor efficiency compared with wild-type protein. After starting thyroxine treatment, plasma cholesterol almost normalized. CONCLUSIONS: This case illustrates the necessity of measuring both FT4 and TSH when hypothyroidism is suspected, or needs to be ruled out. In addition, this case suggests that the presence of childhood hypercholesterolemia may be an indication of undiagnosed hypothyroidism.

Mutations in TBL1X Are Associated With Central Hypothyroidism.

CONTEXT: Isolated congenital central hypothyroidism (CeH) can result from mutations in TRHR, TSHB, and IGSF1, but its etiology often remains unexplained. We identified a missense mutation in the transducin ß-like protein 1, X-linked (TBL1X) gene in three relatives diagnosed with isolated CeH. TBL1X is part of the thyroid hormone receptor-corepressor complex. OBJECTIVE: The objectives of the study were the identification of TBL1X mutations in patients with unexplained isolated CeH, Sanger sequencing of relatives of affected individuals, and clinical and biochemical characterization; in vitro investigation of functional consequences of mutations; and mRNA expression in, and immunostaining of, human hypothalami and pituitary glands. DESIGN: This was an observational study. SETTING: The study was conducted at university medical centers. PATIENTS: Nineteen individuals with and seven without a mutation participated in the study. MAIN OUTCOME MEASURES: Outcome measures included sequencing results, clinical and biochemical characteristics of mutation carriers, and results of in vitro functional and expression studies. RESULTS: Sanger sequencing yielded five additional mutations. All patients (n = 8; six males) were previously diagnosed with CeH (free T4 [FT4] concentration below the reference interval, normal thyrotropin). Eleven relatives (two males) also carried mutations. One female had CeH, whereas 10 others had low-normal FT4 concentrations. As a group, adult mutation carriers had 20%-25% lower FT4 concentrations than controls. Twelve of 19 evaluated carriers had hearing loss. Mutations are located in the highly conserved WD40-repeat domain of the protein, influencing its expression and thermal stability. TBL1X mRNA and protein are expressed in the human hypothalamus and pituitary. CONCLUSIONS: TBL1X mutations are associated with CeH and hearing loss. FT4 concentrations in mutation carriers vary from low-normal to values compatible with CeH.

A specific mutation in TBL1XR1 causes Pierpont syndrome.

BACKGROUND: The combination of developmental delay, facial characteristics, hearing loss and abnormal fat distribution in the distal limbs is known as Pierpont syndrome. The aim of the present study was to detect and study the cause of Pierpont syndrome. METHODS: We used whole-exome sequencing to analyse four unrelated individuals with Pierpont syndrome, and Sanger sequencing in two other unrelated affected individuals. Expression of mRNA of the wild-type candidate gene was analysed in human postmortem brain specimens, adipose tissue, muscle and liver. Expression of RNA in lymphocytes in patients and controls was additionally analysed. The variant protein was expressed in, and purified from, HEK293 cells to assess its effect on protein folding and function. RESULTS: We identified a single heterozygous missense variant, c.1337A>G (p.Tyr446Cys), in transducin ß-like 1 X-linked receptor 1 (TBL1XR1) as disease-causing in all patients. TBL1XR1 mRNA expression was demonstrated in pituitary, hypothalamus, white and brown adipose tissue, muscle and liver. mRNA expression is lower in lymphocytes of two patients compared with the four controls. The mutant TBL1XR1 protein assembled correctly into the nuclear receptor corepressor (NCoR)/ silencing mediator for retinoid and thyroid receptors (SMRT) complex, suggesting a dominant-negative mechanism. This contrasts with loss-of-function germline TBL1XR1 deletions and other TBL1XR1 mutations that have been implicated in autism. However, autism is not present in individuals with Pierpont syndrome. CONCLUSIONS: This study identifies a specific TBL1XR1 mutation as the cause of Pierpont syndrome. Deletions and other mutations in TBL1XR1 can cause autism. The marked differences between Pierpont patients with the p.Tyr446Cys mutation and individuals with other mutations and whole gene deletions indicate a specific, but as yet unknown, disease mechanism of the TBL1XR1 p.Tyr446Cys mutation.

Spatial and temporal expression of immunoglobulin superfamily member 1 in the rat.

Loss-of-function mutations in the immunoglobulin superfamily member 1 (IGSF1) gene cause an X-linked syndrome of central hypothyroidism, macroorchidism, variable prolactin and GH deficiency, delayed pubertal testosterone rise, and obesity. To understand the pathophysiology of this syndrome, knowledge on IGSF1's place in normal development is imperative. Therefore, we investigated spatial and temporal protein and mRNA expression of IGSF1 in rats using immunohistochemistry, real-time quantitative PCR (qPCR), and in situ hybridization. We observed high levels of IGSF1 expression in the brain, specifically the embryonic and adult choroid plexus and hypothalamus (principally in glial cells), and in the pituitary gland (PIT1-lineage of GH, TSH, and PRL-producing cells). IGSF1 is also expressed in the embryonic and adult zona glomerulosa of the adrenal gland, islets of Langerhans of the pancreas, and costameres of the heart and skeletal muscle. IGSF1 is highly expressed in fetal liver, but is absent shortly after birth. In the adult testis, IGSF1 is present in Sertoli cells (epithelial stages XIII-VI), and elongating spermatids (stages X-XII). Specificity of protein expression was corroborated with Igsf1 mRNA expression in all tissues. The expression patterns of IGSF1 in the pituitary gland and testis are consistent with the pituitary hormone deficiencies and macroorchidism observed in patients with IGSF1 deficiency. The expression in the brain, adrenal gland, pancreas, liver, and muscle suggest IGSF1's function in endocrine physiology might be more extensive than previously considered.