THE EFFECTS OF PHENYLBUTAZONE ON THYROID FUNCTION

Abstract

ABSTRACT It is known that phenylbutazone suppresses the thyroidal uptake of radioactive iodine and the serum level of protein-bound iodine (PBI) during the first week of treatment, with a return to normal values after 2 weeks of continuous treatment. Up till now the initial suppression of thyroid function has been presumed due to inhibition of TSH secretion. In the present study, total serum thyroxine and percentage dialysable thyroxine have been measured and the serum absolute free thyroxine concentration calculated in 12 patients before starting treatment with phenylbutazone orally, after 4 days of treatment and again after 14 days. Serum TSH was assayed in 10 of these patients before, and on the 4th day of treatment. Sera were assayed for TSH after 14 days on the drug in 6 patients. On the 4th day of treatment, the levels of total thyroxine had fallen but the levels of free thyroxine remained unchanged. TSH levels were also unaltered. By the 14th day of treatment, free thyroxine levels had fallen significantly below pre-treatment values but no significant rise in TSH could be demonstrated in the 6 patients studied. At no time was there a fall in TSH levels and we conclude that suppression of some thyroid function tests during the first week of treatment with phenylbutazone is due to direct inhibition of the gland by the drug.