G Van Vliet

Development of the thyroid gland: lessons from congenitally hypothyroid mice and men.

Congenital hypothyroidism is the most common congenital endocrine disorder (one newborn in 3000) and represents the most common cause of preventable mental retardation. In 10–20% of cases, it is due to autosomal recessive functional disorders leading to goiter formation (thyroid dyshormonogenesis). In the remainder, it is due to thyroid dysgenesis, which comprises usually isolated defects in: (1) migration of the median thyroid anlage, leading to a round cluster of ectopic cells (usually in a sublingual position) with no other thyroid tissue present; (2) differentiation or survival of the thyroid follicular cells leading to athyreosis; and (3) growth of a thyroid with the normal bilobed shape and in the normal cervical position (orthotopic hypoplasia). Mouse knock‐outs have demonstrated that thyroid transcription factor‐1 (TTF‐1) and PAX8 are required for the survival and proliferation of thyroid follicular cell precursors, TTF‐2 for their downward migration and the thyrotropin receptor (TSHR) for post‐natal thyroid growth. In humans, thyroid dysgenesis is generally a sporadic malformation but an affected relative is found in 2% of cases, a figure 15‐fold higher than by chance alone. Pedigree analysis is most compatible with dominant inheritance with variable penetrance. However, mutations in TTF‐1, TTF‐2, PAX8 and TSHR are found in <10% of patients with congenital hypothyroidism and these predominantly have orthotopic thyroid hypoplasia, often associated with other malformations. This low yield and the discordance of >90% of monozygotic twin pairs suggests that isolated thyroid ectopy or athyreosis most often results from early somatic mutations, epigenetic modifications or stochastic developmental events.

Prevention of intellectual disability through screening for congenital hypothyroidism: how much and at what level?

Objective Congenital hypothyroidism (CHT) is a common cause of preventable mental retardation, and the quantification of intellectual disability due to CHT is needed to assess the public health benefit of newborn screening. Design Review of published studies conducted among children born prior to the introduction of newborn screening for CHT and reporting cognitive test scores. Setting Population-based studies. Patients Children with clinically diagnosed CHT. Interventions Thyroid hormone substitution. Main outcome measures Intelligence quotient (IQ) (mean and distribution). Results The prevalence of recognised CHT rose from one in 6500 prior to screening to approximately one in 3000 with screening. In four population-based studies in high-income countries, among children with clinically diagnosed CHT 8–28% were classified as having intellectual disability (defined as an IQ <70) and the mean IQ was 85 (a leftward shift of 1 SD). Among children with subclinical CHT, the risk of overt intellectual disability was lower (zero in one study), but decreased intellectual potential and increased behavioural abnormalities were documented. Conclusions Although the prevalence of overt disability among children with CHT in the absence of screening may be less than previously estimated, the preventable burden of intellectual disability due to CHT is substantial and justifies newborn screening. However, changes in existing newborn screening protocols to capture more cases are unlikely to prevent overt cases of disability and should therefore be justified instead by the documentation of other benefits of early detection.

Androgens and fetal growth

Boys are heavier than girls at term birth. Children with a 46,XY karyotype and androgen insensitivity syndrome (clinically complete form and/or proven mutations in the androgen receptor gene) were found to have a birth weight comparable to that of girls. These findings support the hypothesis that the difference in birth weight between boys and girls is generated by androgen action.

IGF-I transcript levels in whole-liver tissue, in freshly isolated hepatocytes, and in cultured hepatocytes from lean and obese Zucker rats

Background: The mechanisms underlying the maintenance of normal to high rates of linear growth and plasma insulin-like growth factor I (IGF-I) levels in spite of a low growth hormone secretion in obese children remain unknown. Among the animal models of early-onset obesity, obese Zucker (fa/fa) rats (which are homozygous for an inactivating missense mutation in the leptin receptor) are particularly appropriate, because their linear growth shows this growth hormone independence. Methods: To study the regulation of IGF-I synthesis in this model, we have established primary cultures of hepatocytes derived from 12-week-old Zucker male obese and lean rats. The rat IGF-I gene contains six exons, and alternative splicing generates different mRNAs, one of which (called IGF-1B) has been shown to be decreased by fasting. We report steady state mRNA levels for IGF-I (all transcripts) and for IGF-IB in hepatocytes after 3 days in culture, in freshly isolated hepatocytes, and in whole-liver tissue. RT-PCRs using primers specific for IGF-I or IGF-IB were performed with two different internal competitors for quantification. Results: In primary cultures of hepatocytes, the IGF-IB mRNA was increased by >50-fold (p = 0.01) in cells derived from obese animals as compared with cells from lean animals. However, these transcript levels were not significantly different when measured in freshly isolated hepatocytes or in whole-liver tissue. Conclusions: Increased IGF-IB transcription could be an intrinsic characteristic of cultured hepatocytes harbouring leptin receptors that bear the fa mutation. However, the modulation of this characteristic by cell-cell interactions and by in vivo hormone and metabolic status remains to be studied.

Molecular studies of a patient with complete androgen insensitivity and a 47,XXY karyotype.

A phenotypic female with complete androgen insensitivity from a maternally inherited mutation in the androgen receptor had a 47,XXY karyotype. Partial maternal X isodisomy explained the expression of androgen insensitivity despite the presence of 2 X chromosomes.

Effects of early high-dose levothyroxine treatment on auditory brain event- related potentials at school entry in children with congenital hypothyroidism

Aims: We tested whether brain event-related potentials (ERPs) are normal in children with congenital hypothyroidism (CH) after early high-dose levothyroxine treatment. Methods: Auditory ERPs were recorded in 33 normal controls and in 15 children with CH at 5 years 9/12. Based on bone maturation at diagnosis, the CH group was divided into severe (n = 8) and moderate (n = 7) subgroups. CH patients were treated at a median age of 14 days with a mean initial dose of levothyroxine of 11.6 µg/kg·day. Two ERP components (N100 and N200) were measured and clinical follow-up variables collected. Results: The functional anatomical and cognitive organisation of the auditory system, as revealed by the analyses of ERP measures, did not differ between CH and controls, or between severe and moderate CH subjects. However, N200 latency was globally longer in the CH than in the control group (p = 0.01) and was positively correlated with the over-treatment index (r = 0.61; p < 0.05) and verbal IQ. N200 amplitude was negatively correlated with initial dose (r = –0.74; p < 0.005). Conclusion: These data suggest that sensitive tools such as ERPs can reveal differences between CH and controls and relate these differences to the adequacy of treatment of CH.

The growth hormone (GH)-independent growth of the obese Zucker rat is not due to increased levels of GH receptor messenger RNA in the liver

Obese Zucker rats maintain normal rates of linear growth and circulating concentrations of insulin-like growth factor-I (IGF-I) and of IGF-binding protein-3 (IGFBP-3) in spite of low GH secretion. The mechanisms underlying this GH-independent growth in obesity are unknown. To assess whether the liver expression of the GH receptor (GHR) messenger RNA (mRNA) is increased and/or if the liver expression of IGFBP-3 mRNA is maintained in the obese, Zucker rats of both genders and phenotypes (four groups, n = 6/group) were studied at 12 weeks of age. By Northern analysis, mRNA levels for GHR and GHBP were not increased in obese rats compared to their sex-matched lean littermates; the expression of these two transcripts was sexually dimorphic and the changes in GHBP mRNA/GHR mRNA ratios associated with obesity were sex-specific. In both genders, IGFBP-1 and IGFBP-3 mRNAs were decreased in the obese. We concluded that the GH-independent growth of obese Zucker rats is not due to increased GHR mRNA or to maintained IGFBP-3 mRNA levels in the liver.