Maternal thyroid hormones during pregnancy: effects on the fetus in congenital hypothyroidism and in iodine deficiency

Deiodinase-mediated thyroid hormone inactivation minimizes thyroid hormone signaling in the early development of fetal skeleton

Thyroid hormone (TH) plays a key role on post-natal bone development and metabolism, while its relevance during fetal bone development is uncertain. To study this, pregnant mice were made hypothyroid and fetuses harvested at embryonic days (E) 12.5, 14.5, 16.5 and 18.5. Despite a marked reduction in fetal tissue concentration of both T4 and T3, bone development, as assessed at the distal epiphyseal growth plate of the femur and vertebra, was largely preserved up to E16.5. Only at E18.5, the hypothyroid fetuses exhibited a reduction in femoral type I and type X collagen and osteocalcin mRNA levels, in the length and area of the proliferative and hypertrophic zones, in the number of chondrocytes per proliferative column, and in the number of hypertrophic chondrocytes, in addition to a slight delay in endochondral and intramembranous ossification. This suggests that up to E16.5, thyroid hormone signaling in bone is kept to a minimum. In fact, measuring the expression level of the activating and inactivating iodothyronine deiodinases (D2 and D3) helped understand how this is achieved. D3 mRNA was readily detected as early as E14.5 and its expression decreased markedly ( approximately 10-fold) at E18.5, and even more at 14 days after birth (P14). In contrast, D2 mRNA expression increased significantly by E18.5 and markedly ( approximately 2.5-fold) by P14. The reciprocal expression levels of D2 and D3 genes during early bone development along with the absence of a hypothyroidism-induced bone phenotype at this time suggest that coordinated reciprocal deiodinase expression keeps thyroid hormone signaling in bone to very low levels at this early stage of bone development.

Thyroid hormone receptor beta is essential for development of auditory function

Congenital thyroid disorders are often associated with profound deafness, indicating a requirement for thyroid hormone (T3) and its receptors in the development of hearing. Two T3 receptor genes, Tr alpha and Tr beta are differentially expressed, although in overlapping patterns, during development. Thus, the extent to which they mediate unique or redundant functions is unclear. We demonstrate that Tr beta-deficient (Thrb-/-) mice exhibit a permanent deficit in auditory function across a wide range of frequencies, although they show no other overt neurological defects. The auditory-evoked brainstem response (ABR) in Thrb-/- mice, although greatly diminished, displayed normal waveforms, which suggested that the primary defect resides in the cochlea. Although hypothyroidism causes cochlear malformation, there was no evidence of this in Thrb-/- mice. These findings suggest that Tr beta controls the maturation of auditory function but not morphogenesis of the cochlea. Thrb-/- mice provide a model for the human endocrine disorder of resistance to thyroid hormone (RTH), which is typically associated with dominant mutations in Tr beta. However, deafness is generally absent in RTH, indicating that dominant and recessive mutations in Tr beta have different consequences on the auditory system. Our results identify Tr beta as an essential transcription factor for auditory development and indicate that distinct Tr genes serve certain unique functions.

Endemic goiter and thyroid function in central-southern Sardinia. Report on an extensive epidemiological survey

Although the existence of endemic goiter and cretinism in Sardinia is known since to ancient time, scanty information collected according to WHO criteria is available. In the present paper the results of an extensive epidemiological survey carried out in juvenile population living in some rural and/or hilly villages in the provinces of Nuoro and Oristano in Central-Southern Sardinia and in urban area of Cagliari, are reported. In the majority of the villages the mean urinary iodine excretion was lower than 60 micrograms/L; the goiter prevalence ranged between 39% and 61% in the district of Nuoro and between 21% and 56% in the district of Oristano. In the control area the urinary iodine excretion was 105 micrograms/L with a goiter prevalence of 12%. Goiter prevalence was not always inversely related to urinary iodine excretion. No relevant thyroid function alterations were found.

IN CONCLUSION

1) in extraurban areas of Central-Southern Sardinia mild to moderate iodine deficiency and endemic goiter are still a widespread problems; 2) also in urban area endemic goiter prevalence is still higher than 10%; 3) extemporary urinary samples are inadequate for assessing the severity of goiter endemia in mild to moderate iodine deficiency; 4) in mildly affected districts palpation is inaccurate for assessing the prevalence of goiter; 5) no relevant alterations of thyroid function were documented in juvenile population.