Nodular hyperplasia of thyroid glands induced by thiouracil.

Summary The changes in weight and morphology of the thyroid gland and hypophysis following chronic administration of thiouracil in the diet have been described. Analysis of the data has shown that a progressive increase in weight of both organs was limited to the female rats. There was evidence of involution of the thyroid gland in male rats which had received the diet for 10 months. The regressive changes in the thyroid gland were associated with cytologic alterations in the hypophyses, and these have been discussed briefly. Diffuse hyperplasia of the thyroid gland was the most uniform response to the drug. In 33 per cent of all rats, definite nodular hyperplasia was found. The histologic structure of the nodules varied, and the same gland harbored nodules of different histologic appearance in several instances. All nodules were regarded as benign lesions. Tubular proliferation of the epithelial cells lining the acini was seen in 40 per cent of the rats. Examination of small foci of nodular hyperplasia failed to demonstrate a relationship between the tubules and the nodules, and both changes were seen in the same gland as distinctly separate structures. Ultimo-branchial tissue was found in 12 per cent of the animals. Early lesions of nodular or tubular hyperplasia did not show a definite association with ultimo-branchial tissue such as has been found in spontaneous tumors of the thyroid gland in the rat. Factors which may contribute to the development of nodular hyperplasia are discussed briefly in the light of comparable lesions in other endocrine organs and different species.