ExtractThyroidectomies were performed through a flank incision and uterotomy on five fetuses of 1-4-year-old Columbia and Columbia Suffolk date-bred ewes of 90-1 1 0 days gestation. The fetuses were killed 19-43 days post-thyroidectomy. Body and organ weights, limb roentgenograms, DNA, RNA, and protein concentrations of selected tissues, cerebral and cerebellar lipid concentrations, and cerebellar cerebroside fatty acid composition were measured and compared with similar measurements in control animals. In the thyroidectomized fetuses mean carcass and lung weights were significantly reduced and wool development was consistently delayed. Extremity x-rays showed a delay in the time of appearance and a decrease in the size of epiphyseal centers. The mean DNA concentration was low in muscle tissue; mean protein concentrations were reduced in cerebellum, heart, lung, thymus, and muscle tissues, while mean total lipid concentration was low in cerebral tissue; the percentage of 18-carbon fatty acids was significantly increased in cerebellar cerebrosides. These results indicate that thyroid hormone deficiency, present during the last trimester of gestation in the ovine fetus, impairs carcass and lung growth, delays bone and skin maturation, inhibits growth in cell size in heart, lung, thymus, and cerebral tissues, and delays myelination in the central nervous system (CNS).
SpeculationThe present results, considered with results of other investigations of the effects of thyroid hormone deficiency in fetal and newborn mammals, indicate that hypothyroidism impairs somatic growth, delays bone growth and maturation, delays cell growth and replication in the CNS, and inhibits CNS myelination. The critical period for these effects varies in different species. In the rat this period is postnatal, whereas in the sheep it extends into the third trimester of pregnancy. During pregnancy, carcass growth, bone maturation, and lung maturation are most affected in the sheep; CNS changes are minimal. In man fetal hypothyroidism may produce some delay in bone maturation but somatic growth retardation and signs and symptoms of hypothyroidism are not usually present a t birth. Moreover, as in the sheep, any delay in CNS maturation probably is minimal in utero, since very early postnatal treatment minimizes mental retardation. Thus early diagnosis and treatment of hypothyroidism is necessary and consideration of the possibility of newborn screening would seem appropriate.19, 23). Therefore, it is possible that thyroid hormones are not necessary for normal fetal somatic growth. The effect of intrauterine hypothyroidism on CNS growth and maturation also is not entirely clear. Thus we have conducted studies of body and organ growth and thyroid hormone kinetics in the fetal sheep after thyroidectomy. The kinetic studies, described elsewhere (141, indicated that fetal serum levels of both thyroxine (T4) and triiodothyronine ( T 3 ) are unmeasurable soon after fetal thyroidectomy and that the athyrotic fetus receives minimal quantities of materna...
