The Effect of an Oral Contraceptive on Tests of Thyroid Function

Abstract

To determine the effect of a combined oral progestin on 5 tests of thyroid function, 21 parous women at least 8 weeks postpartum and with histories of regular menses were studied. A complete physical examination showed all to be normal. The 5 tests performed were radioactive iodine uptake (RAI) at 2 and 24 hours, serum protein-bound iodine (PBI), thyroxine iodine by column, triiodothyronine absorption test, and serum cholesterol. 2 baseline determinations of each test except the RAI were performed on each subject on separate days. Only euthyroid subjects were further tested. Of these 16 were given 10 mg of medroxyprogesterone acetate in combination with .05 mg of ethinyl estradiol cyclically for 20 days. Thyroid function tests were repeated at various intervals from the end of the first week of therapy to over 4 months after starting therapy. Cholesterol and RAI determinations were extremely variable precluding any evidence of drug effect. The other 3 tests showed consistent changes in all patients studied. The serum PBI and thyrozine-iodine by column tests both showed slight elevation within the first week of therapy and further elevation 1 months thereafter. These changes approached hyperthyroidism levels. The triiodothyronine absorption test showed little change in the first week but a definite downward shift thereafter with a maximum depression at 3 months of therapy. This change reached hypothyroidism level. If test were done during the 1 week each month patients were not taking the drug, results were the same. These changes are thought to be due to the estrogen component of the contraceptive drugs. Those physicians depending on these thyroid tests for diagnosis should be aware of these changes in patients taking these drugs.