Early development in the caudate and thyroid of methimazole-treated rats.

Rat pups were made hypothyroid by administration of the antithyroid agent, methimazole, to pregnant and lactating dams from gestational day 17 to postnatal day 10. Dendritic branching of caudate neurons in methimazole-treated pups was decreased 30 to 40 percent between 1 and 28 days of age. Perinatal methimazole treatment produced a pattern of developmental deficit or prolonged delay in the growth of dendrites of caudate interneurons. The antithyroid effect was also apparent in morphometric changes in the thyroid gland of methimazole-treated rats. Functional tissue was present at all ages examined. Colloid storage in the thyroid was decreased during methimazole administration compared with colloid storage after methimazole was discontinued. Both during and after methimazole administration the size of thyroid follicular cell nuclei in methimazole-treated pups was significantly greater than that of thyroid nuclei in control animals. Changes in dendritic branching, body weight and thyroxine levels lasted into adult life but recovery toward control levels was seen in adults. These changes in offspring following maternal administration of a goitrogen were demonstrated at an exposure level well below complete thyroid suppression.