[Changes in thyroid function caused by continuous treatment with thiouracil in rats]

Serum concentration evolution of thyroxine and triiodothyronine, and thyrotropin (TSH), have been studied in rats while they were given 6-propylthiouracil (PTU) as antithyroid drug, and during the recovery period after suppression of treatment. In the same way thyroid hypertrophy and plasmatic levels of thyrotropin were correlated. Animals received orally a daily dose of 1 mg/100 g body weight during thirty-five days and had a two week recovery period. Thyroid hormone concentrations in plasma were determined by immunoenzymatic assay ELISA with peroxidase as labelled enzyme. From the results obtained, it can be stated that chronic administration of PTU implies a continuous decrease in thyroid hormone concentrations in plasma, reaching nearly zero values, while after treatment, levels recover their normal values in a week's time. A parallelism exists between thyroid hypertrophy and pituitary TSH hypersecretion, due to a decrease in thyroid hormone levels.