Thyroid suppression tests during drug treatment of hyperthyroidism.

In fifteen hyperthyroid patients treated for 5 or more consecutive years with propylthiouracil four or more yearly prognostic 'suppression' tests (Cassidy & VanderLaan, 1960) were done. In nine of the fifteen patients there was a progressive diminution over a period of 3-6 years in the values of these suppressed uptakes to 20% or below. Eight of these nine patients have remained euthyroid off therapy for 1-8 years (average 3-1 years). None have become hypothyroid. The data suggest a gradual continuing change in thyroid function over the course of years in some hyperthyroid patients on antithyroid therapy. This may be a long-term effect of the drug and may have prognostic significance.