Nadia Schoenmakers

Objective: Restoration of euthyroidism in patients with thyrotoxicosis prior to thyroidectomy is recommended to decrease perioperative morbidity and thyroid storm risk. This is challenging when conventional anti-thyroid medication is contraindicated or ineffective. Iopanoic acid (IOPA), an iodinated contrast medium formerly used as an oral cholecystographic agent, may facilitate treatment of thyrotoxicosis, but limited manufacture precludes its use in the UK. We investigated the effectiveness and safety of IOPA for optimizing thyroid status in treatment-resistant thyrotoxicosis prior to thyroidectomy at a referral centre. Methods: With prior permission of our Drug and Therapeutics Committee, we sourced laboratory-grade (>98% pure) IOPA and administered this orally to control ongoing, severe thyrotoxicosis prior to thyroidectomy in 12 patients with inadequate response to standard therapies. Underlying aetiologies included Graves’ disease, amiodarone-induced thyrotoxicosis, toxic multinodular goitre and resistance to thyroid hormone beta. Medical case notes were reviewed retrospectively to analyse clinical and biochemical outcomes. Results: All patients exhibited a decline and normalization/near-normalization of free T3 (FT3) levels (mean 55% decrease, SEM 4.5%), with minimal changes in FT4 levels. Eleven patients proceeded to uneventful thyroidectomy, and IOPA was well tolerated with no directly attributable side effects. Conclusion: IOPA is a safe and effective agent for controlling biochemical thyrotoxicosis in preparation for thyroidectomy, including disease refractory to conventional medication. Highly pure, laboratory-grade iopanoic acid that is approved for human use is now available in the UK; consideration should be given to more widespread use for emergency treatment of life-threatening hyperthyroidism.

Primary congenital hypothyroidism (CH) is the most common neonatal endocrine disorder, and may be etiologically subdivided into thyroid dysgenesis, referring to abnormal thyroid development, and dyshormonogenesis, where a defective thyroid hormone biosynthesis pathway results in inadequate hormone production despite a structurally intact gland. Delayed treatment of neonatal hypothyroidism may result in irreversible neurodevelopmental impairment; therefore, where available, CH screening programs facilitate prompt diagnosis. However, the molecular basis for CH remains unclear in most of the cases. This review summarizes current understanding of the genetic etiologies underlying primary CH and associated phenotypes. Classical genetic causes are discussed in the context of their role in normal thyroid physiology. Genes recently reported to play a role in the pathogenesis of CH are discussed, and novel genomic mechanisms in CH are described.

Introduction: Paired box gene 8 (PAX8) is a key transcription factor required both for normal fetal thyroid development and maintenance of the differentiated thyroid phenotype, mediating transcriptional activation of SLC5A5, TG, and TPO, and synergizing with NKX2-1 at the TG promoter. Heterozygous PAX8 mutations are a rare but well-recognized cause of congenital hypothyroidism (CH) due to thyroid dysgenesis (TD), and are classically associated with thyroid hypoplasia. However, a spectrum of thyroidal morphologies and biochemical severities have been reported, with infrequent association of urogenital tract malformations. We report diverse clinical phenotypes in 4 kindreds harbouring different heterozygous PAX8 mutations, with supporting laboratory functional characterization of the mutant PAX8. Case Reports: P1 harbours a novel PAX8 missense mutation (p.I34F) associated with severe CH, thyroid hypoplasia and hypospasias. P2 is a female child harbouring a truncating mutation (p.R207*) previously reported in the context of Mayer-Rokitansky-Kuster-Hauser (MRKH) syndrome. Here, p.R207* is newly associated with mild CH, and absent Tc-99m uptake in an ultrasonographically normal sized thyroid gland-in-situ (GIS) without associated urogenital tract defects. P3 exhibits severe CH with a dysplastic thyroid associated with PAX8 c.162C>A, p.S54R; the same amino acid mutation due to a different nucleotide change (c.162C>G) has previously been reported in individuals with TD. P4a and P4b are sisters harbouring PAX8 p.S59R associated either with permanent GIS CH or mild CH with hemiagenesis. This contrasts with previously reported cases in whom p.S59R is associated with severe GIS CH and partial iodide organification defect, or goitrous CH with cryptorchidism and hydrocele. Laboratory Data: p.S54R has previously been characterized in detail (1). Here, functional evaluation of PAX8 p.S59R, p.R207* and p.I34F demonstrated that the mutant proteins were expressed, but exhibited impaired transactivation of both TPO and TG promoters in luciferase reporter assays, compared to wild-type PAX8. Co-transfection studies with NKX2-1 achieved at least partial rescue of TG promoter transactivation with both missense mutations but not p.R207*, supporting a role for the PAX8 carboxyterminus in this interaction. Homology modelling suggests steric hindrance due to the p.S59R and p.I34F mutations may indirectly affect PAX8-DNA interactions. Conclusion: Our studies characterize a novel PAX8 mutation (p.I34F) associated with both thyroid and urogenital tract pathology, and yield further insights into PAX8-NKX2-1 synergism. Additionally, we expand the clinical phenotypes associated with reported PAX8 mutations, demonstrating isolated CH with possible impaired SLC5A5 function in MRKH-associated p.R207*, and thyroid hemiagenesis with p. S59R. 1. Hermanns et al (2013) Thyroid 23, 791

Normal functioning of the thyroid is of profound importance for lifetime health due to its role in hormone production. Dysfunction of the thyroid is associated with severe congenital pathologies, some of them appearing in childhood. For example, over half the babies born with congenital hypothyroidism appear completely normal and have no symptoms. However, early diagnosis of thyroid defects is lacking mainly due to a poor understanding of the development of the tissue in utero. Here we have established a comprehensive spatiotemporal atlas of the developing human thyroid during the first and second trimester of pregnancy. Our dense profiling of more than 100k cells using single-cell sequencing has revealed the main cell types, their developmental relationships and transcription factors leading to the formation of the thyroid gland. Notably, we found that thyrocytes are heterogeneous epithelial populations and split thyroid-hormones production between different subsets. We further validated the spatial heterogeneity of thyrocyte subpopulations using multiple spatial transcriptomics methods. Our results confirm the division of labour of the thyrocytes, highlighting functional specialisation amongst them. Altogether our analysis exemplifies the division of labour principle observed in other adult tissues also applies to the development of the thyroids, expanding our knowledge of thyroid-hormones synthesis and regulation. Future work includes how the function principles are altered in pathological conditions.

Development and differentiation of the thyroid gland is directed by expression of specific transcription factors in the thyroid follicular cell which mediates hormone biosynthesis. Membrane transporters are rate‐limiting for cellular entry of thyroid hormones (TH) (T4 and T3) into some tissues, with selenocysteine‐containing, deiodinase enzymes (DIO1 and DIO2) converting T4 to the biologically active hormone T3. TH regulate expression of target genes via hormone‐inducible nuclear receptors (TRα and TRβ) to exert their physiological effects. Primary congenital hypothyroidism (CH) due to thyroid dysgenesis may be mediated by defects in thyroid transcription factors or impaired thyroid stimulating hormone receptor function. Dyshormonogenic CH is usually due to mutations in genes mediating thyroidal iodide transport, organification or iodotyrosine synthesis and recycling. Disorders of TH signalling encompass conditions due to defects in membrane TH transporters, impaired hormone metabolism due to deficiency of deiodinases and syndromes of Resistance to thyroid hormone due to pathogenic variants in either TRα or TRβ. Here, we review the genetic basis, pathogenesis and clinical features of congenital, dysgenetic or dyshormonogenic hypothyroidism and disorders of TH transport, metabolism and action.

Transient congenital hypothyroidism (TCH) refers to congenital hypothyroidism which spontaneously resolves in the first few months or years of life. Currently, there is a paucity of reliable markers predicting TCH at diagnosis, and the diagnosis is established following the withdrawal of levothyroxine therapy around 3 years of age. The incidence of TCH is increasing, and it is a major contributor to the overall increase in the incidence of CH in recent studies. Both genetic factors, in particular mutations affecting DUOX2 and DUOXA2, and environmental factors, for example, iodine deficiency and excess, anti- TSHR antibodies and exposure to antithyroid or iodine-rich medications, may cause TCH. Resolution of TCH in childhood may reflect both normal thyroid physiology (decreased thyroid hormone biosynthesis requirements after the neonatal period) and clearance or cessation of environmental precipitants. The relative contributions and interactions of genetic and environmental factors to TCH, and the extent to which TCH may be prevented, require evaluation in future population-based studies. Abstract Transient congenital hypothyroidism (TCH) refers to congenital hypothyroidism which spontaneously resolves in the first few months or years of life. Currently, there is a paucity of reliable markers predicting TCH at diagnosis, and the diagnosis is established following the withdrawal of levothyroxine therapy around 3 years of age. The incidence of TCH is increasing, and it is a major contributor to the overall increase in the incidence of CH in recent studies. Both genetic factors, in particular mutations affecting DUOX2 and DUOXA2, and environmental factors, for example, iodine deficiency and excess, anti- TSHR antibodies and exposure to antithyroid or iodine-rich medications, may cause TCH. Resolution of TCH in childhood may reflect both normal thyroid physiology (decreased thyroid hormone biosynthesis requirements after the neonatal period) and clearance or cessation of environmental precipitants. The relative contributions and interactions of genetic and environmental factors to TCH, and the extent to which TCH may be prevented, require evaluation in future population-based studies.

Congenital Hypothyroidism (CH) due to dyshormonogenesis may occur due to mutations in any of the key genes involved in thyroid hormone biosynthesis (TG, TPO, DUOX2, DUOXA2, SLC5A5, IYD, SLC26A4 and SLC26A7). Mutations in the Thyroglobulin gene (TG) are frequently associated with goiter, which may present fetally or neonatally, although a spectrum of phenotypes is reported. We present the case of a woman of Eritrean origin who presented in the third trimester of pregnancy in the early stages of labor. Ultrasound at presentation revealed a fetal neck swelling consistent with a goiter. Following delivery by caesarian section with minimal respiratory support, the infant was found to be hypothyroid with undetectable serum levels of thyroglobulin. Sequencing of the thyroglobulin gene revealed a homozygous donor splice site pathogenic variant (c.5686+1delG) not previously described in the literature. Levothyroxine treatment resulted in normal growth and psychomotor development. Goitrous CH with inappropriately low thyroglobulin has previously been reported in patients harbouring homozygous single nucleotide substitutions at the same TG donor splice site which result in exon skipping and Endoplasmin Reticulum retention of TG. We conclude that the TG c.5686+1delG pathogenic variant is the likely basis for our patient's fetal goiter and congenital hypothyroidism, and that the clinical phenotype associated with TG c.5686+1delG is comparable to that seen with single nucleotide substitutions at the same site.

The sodium-iodide symporter (NIS, SLC5A5) is expressed at the basolateral membrane of the thyroid follicular cell, and facilitates the thyroidal iodide uptake required for thyroid hormone biosynthesis. Biallelic loss-of-function mutations in NIS are a rare cause of dyshormonogenic congenital hypothyroidism. Affected individuals typically exhibit a normally sited, often goitrous thyroid gland, with absent uptake of radioiodine in the thyroid and other NIS-expressing tissues. We report a novel homozygous NIS mutation (c.1067 C>T, p.S356F) in four siblings from a consanguineous Indian kindred, presenting with significant hypothyroidism. Functional characterization of the mutant protein demonstrated impaired plasma membrane localization and cellular iodide transport.

Neonatal screening in Macedonia detects congenital hypothyroidism (CH) with an incidence of 1 in 1,585, and more than 50% of cases exhibit a normally located gland-in-situ (GIS). Monogenic mutations causing dyshormonogenesis may underlie GIS CH; additionally, a small proportion of thyroid hypoplasia has a monogenic cause, such as TSHR and PAX8 defects. The genetic architecture of Macedonian CH cases has not previously been studied. We recruited screening-detected, non-syndromic GIS CH or thyroid hypoplasia cases (n = 40) exhibiting a spectrum of biochemical thyroid dysfunction ranging from severe permanent to mild transient CH and including 11 familial cases. Cases were born at term, with birth weight >3,000 g, and thyroid morphologies included goiter (n = 11), thyroid hypoplasia (n = 6), and apparently normal-sized thyroid. A comprehensive, phenotype-driven, Sanger sequencing approach was used to identify genetic mutations underlying CH, by sequentially screening known dyshormonogenesis-associated genes and TSHR in GIS cases and TSHR and PAX8 in cases with thyroid hypoplasia. Potentially pathogenic variants were identified in 14 cases, of which four were definitively causative; we also detected digenic variants in three cases. Seventeen variants (nine novel) were identified in TPO (n = 4), TG (n = 3), TSHR (n = 4), DUOX2 (n = 4), and PAX8 (n = 2). No mutations were detected in DUOXA2, NIS, IYD, and SLC26A7. The relatively low mutation frequency suggests that factors other than recognized monogenic causes (oligogenic variants, environmental factors, or novel genes) may contribute to GIS CH in this region. Future non–hypothesis-driven, next-generation sequencing studies are required to confirm these findings.