HYPERPLASIA AND I131UPTAKE OF THE GUINEA PIG THYROID DURING AND AFTER CHRONIC PROPYLTHIOURACIL TREATMENT*

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Abstract
The chronic admn. of propylthiouracil (0.1% in ration, 2-160 days) to female guinea pigs eventually induced marked thyroid hyperplasia. Histologic alterations were visible after 5-8 days of treatment but goiters did not appear until 23-29 days on the drug regime. Thyroid enlargement thereafter was progressive with 10-fold wt. increase found after 100 days of uninterrupted treatment. The I131 uptake of thyroid tissue (24 hrs. following intraperit admn. of a tracer dose; 8 [mu]c. in 1 ml. soln. without carrier) was decreased in animals receiving goitrogen. The depression in I131 uptake preceded anatomic change in the gland, and was as marked with short as with long periods of treatment. Thyroidal uptakes increased well above normal when the goitrogen was withdrawn from the diet (5 days). The "rebound" in I131 uptake was greater the longer the period of drug treatment, and paralleled the thyroid hyperplasia. The increased avidity for I131 upon drug withdrawal was demonstrable, however, long before morphologic changes appeared in the gland. As a result of increased efficiency and mass of thyroid tissue, I131 collections 10-20 times greater than normal were found in thyroids of animals returned to the normal diet after prolonged treatment with propylthiouracil. The increased avidity of thyroid tissue for I131 after goitrogen withdrawal cannot be entirely accounted for by anatomic alterations, and, rather, may reflect change in some cellular mechanism, presumably enzymatic, in response to prolonged stimulation with endogenous TSH.