Endocrine Abstracts

Thyroid hormone (TH) is essential for the developing nervous system. Early in pregnancy, the maternal thyroid is the fetal brain's sole source of hormone while later, the developing fetal thyroid assumes an increasingly greater role to be fully independent by term. Because brain structures vary as to timing of need for TH, different types of brain impairment and, correspondingly, different neurobehavioral deficits reflect the exact period of TH was insufficient. Several human conditions provide distinct models of TH insufficiencies at different stages of development. In maternal hypothyroidism, there is typically an early TH insufficiency and depending on treatment adequacy, possibly a third trimester insufficiency; in maternal hyperthyroidism, antithyroid medications can block fetal thyroid functioning in later pregnancy; in premature birth, the maternal thyroid supply in the third trimester is severed; and in congenital hypothyroidism (CH), there is a postnatal thyroid hormone insufficiency and, depending on etiology, also a third trimester insufficiency. In my lab, we have been following infants and children with these four conditions. Early studies involved detailed neuropsychological evaluations of several series of children with CH while more recently we have been studying infants and young children with all four clinical conditions. Our methods have involved clinical and experimental tests as well as electrophysiological techniques. Results will be examined in terms of global intelligence and abilities in four domains: visual and visuospatial abilities, sensorimotor skills, attention, and memory. I will show that TH insufficiency in early pregnancy is associated with a different pattern of deficit than thyroid hormone deficiency in mid-to-late pregnancy or postnatally. The particular deficits suggest that in the human, retinal and thalamic development appear to need TH in early pregnancy, visual cortical structures later in pregnancy, while substructures of the hippocampus and cortical association areas need TH postnatally. Experience with late-treated CH cases suggests the frontal lobes need TH later in infancy.