Abstract
Age related spontaneous changes in the thyroid of Sprague-Dawley rats were investigated. Thyroid enlargement and involution, induced by two antithyroid compounds was investigated, using the laboratory rat. In addition, changes induced by phenobarbitone in the thyroid was studied. Standard toxicological parameters, together with hormonal investigation, complemented the results of organ weight analysis and conventional pathological examinations. Quantitative estimation of, the total number of follicles, the number of follicular epithelial cells, proportional volume of follicular epithelial cells and metaphase index, are necessary in order to differentiate hyperplasia from hypertrophy. Propylthiouracil induced hyperplasia of the thyroid. This hyperplasia was shown to be reversible. During the recovery period, single cell degeneration and/or apoptosis were seen in the thyroid, resulting in a reduction in the number of follicular epithelial cells. Potassium perchlorate induced hypertrophy of the follicular epithelial cells. This was shown to be reversible. Phenobarbitone induced hyperplasia of thyroid follicular epithelial cells. This was by indirect mechanism; the compound is believed to act via the liver-thyroid axis. It is possible to give characteristic features to differentiate spontaneous from induced thyroid lesions and hyperplasia from hypertrophy.