Neurological and muscular aspects of thyroid dysfunction in childhood

The effects of thyroid hormone level and action in developing brain: are these targets for the actions of polychlorinated biphenyls and dioxins?

Alterations in thyroid hormone level or responsivity to thyroid hormone have significant neurologic sequelae throughout the life cycle. During fetal and early neonatal periods, disorders of thyroid hormone may lead to the development of motor and cognitive disorders. During childhood and adult life, thyroid hormone is required for neuronal maintenance as well as normal metabolic function. Those with an underlying disorder of thyroid hormone homeostasis or mitochondrial function may be at greater risk for developing cognitive, motor, or metabolic dysfunction upon exposure to substances which alter thyroid hormone economy. Polychlorinated biphenyls (PCBs) and dioxins have been argued to interfere with thyroid hormone action and thus may affect the developing and mature brain. Animal models provide useful tools for studying the effects of thyroid hormone disorders and the effects of environmental endocrine disruptors. The congenitally hypothyroid, hyt/hyt, mouse exhibits abnormalities in both the cognitive and motor systems. In this mouse and other animal models of thyroid hormone disorders, delayed somatic and reflexive development are noted, as are permanent deficits in hearing and locomotor and adaptive motor behavior. This animal's behavioral abnormalities are predicated on anatomic abnormalities in the nervous system. In turn, these abnormalities are correlated with differences in neuronal structural proteins. In normal mice, the expression of mRNAs coding for these proteins occurs temporally with the onset of autonomous thyroid hormone production. The hyt/hyt mouse has a mutation in the thyroid stimulating hormone receptor (TSHr) gene which renders it incapable of transducing the TSH signal in the thyrocyte to produce thyroid hormone. Some behavioral and possibly some biochemical abnormalities in mice exposed to PCBs are similar to those seen in the hyt/hyt mouse. In addition to direct effects on brain development and neuronal maintenance, thyroid hormone is necessary for maintaining metabolic functioning through its influence on mitochondria. Because the brain is particularly sensitive to inadequate energy generation, disorders of thyroid hormone economy also indirectly impair brain functioning. Alterations in thyroid hormone level result in differing expression of mitochondrial genes. Mutations in these mitochondrial genes lead to well-recognized syndromes of encephalomyopathy, myopathy, and multisystem disorder. Hence, PCBs and dioxins, by possibly altering the thyroid hormone milieu, may alter the functioning of mitochondria in the generation of adenosine triphosphate (ATP). The use of animal models of thyroid hormone deficiency for behavioral, anatomic, histologic, and molecular comparison will help elucidate the mechanisms of action of these putative endocrine-disrupting compounds. The study of thyroid hormone disorders provides a template for relating thyroid hormone mediated effects on the brain to these compounds.

Brain magnetic resonance imaging in congenital hypothyroid infants at diagnosis

The aim of our study was to investigate the central nervous system (CNS) morphology and myelination with magnetic resonance imaging (MRI) in congenital hypothyroid (CH) infants detected by neonatal screening before replacement therapy. We studied 11 CH infants, 9 girls and 2 boys, mean age 22 days, 3 with aplasia, 5 with ectopia, 2 with hypoplasia of the thyroid gland, 1 with unknown diagnosis. As normal controls 22 term newborns (38 to 41 weeks of gestational age) were studied. MRI studies were performed with a 1.5-T magnet, extremity coil, T1-weighted and heavily T2-sequences axial sections were obtained. No sedation was needed for the MRI studies. MRI brain examination was normal in all patients compared with controls. In particular, no differences in the myelination patterns of the brain were observed between normal subjects and patients with hypothyroidism. Our study shows no morphological brain abnormalities in CH infants detected by neonatal screening before replacement therapy. Perinatal hypothyroidism seems to have no effect on CNS structures.

Biology of the congenitally hypothyroid hyt/hyt mouse

The hyt/hyt mouse has an autosomal recessive, fetal onset, characterized by severe hypothyroidism that persists throughout life and is a reliable model of human sporadic congenital hypothyroidism. The hypothyroidism in the hyt/hyt mouse reflects the hyporesponsiveness of the thyroid gland to thyrotropin (TSH). This is attributable to a point mutation of C to T at nucleotide position 1666, resulting in the replacement of a Pro with Leu at position 556 in transmembrane domain IV of the G protein-linked TSH receptor. This mutation leads to a reduction in all cAMP-regulated events, including thyroid hormone synthesis. The diminution in T3/T4 in serum and other organs, including the brain, also leads to alterations in the level and timing of expression of critical brain molecules, i.e. selected tubulin isoforms (M beta 5, M beta 2, and M alpha 1), microtubule associated proteins (MAPs), and myelin basic protein, as well as to changes in important neuronal cytoskeletal events, i.e. microtubule assembly and SCa and SCb axonal transport. In the hyt/hyt mouse, fetal hypothyroidism leads to reductions in M beta 5, M beta 2, and M alpha 1 mRNAs, important tubulin isoforms, and M beta 5 and M beta 2 proteins, which comprise the microtubules. These molecules are localized to layer V pyramidal neurons in the sensorimotor cortex, a site of differentiating neurons, as well as a site for localization of specific thyroid hormone receptors. These molecular abnormalities in specific cells and at specific times of development or maturation may contribute to the observed neuroanatomical abnormalities, i.e. altered neuronal process growth and maintenance, synaptogenesis, and myelination, in hypothyroid brain. Abnormal neuroanatomical development in selected brain regions may be the factor underlying the abnormalities in reflexive, locomotor, and adaptive behavior seen in the hyt/hyt mouse and other hypothyroid animals.

Neurological abnormalities in patients treated for hypothyroidism from early life

Neurological and psycholocial assessment was carried out on 30 patients aged 2.7 to 21 years (mean 9.4) who were being treated for hypothyroidism starting before the age of 2 years. Their IQ scores lay in the normal range (71--122; mean 92.4) but 77% showed at least one sign of impaired brain function. Clumsiness was found in 33%, behaviour disorders in 23%, speech disorders in 20%, learning disorders in 26%, squint in 53%, nystagmus in 10%, and minor motor disorders in 50%. Many showed several of these features and hypothyroidism in early life appears to lead to widespeard impairment of brain function. These neurological findings were equally common in patients in whom treatment started between 4 and 10 weeks of age and patients treated after 10 weeks, suggesting that early detection of hypothyroidism by neonatal screening may be of limited benefit. Children who have been hypothyroid in early infancy require careful assessment to prevent further visual, emotional, scholastic, or vocational difficulties.