6-Mercaptopurine (6-MP) has been found to have anti-thyroid properties in rats. Acutely, the drug inhibits thyroid hormone synthesis by interfering with organic iodine binding. The chronic administration of the drug results in inhibition of thyroxine synthesis, increase in plasma TSH and goiter. A state of iodine deficiency which is probably associated with preferential triiodothyronine production by the thyroid and maintenance of normal circulating triiodothyronine levels then results. Goiter and hypothyroidism also have been observed in 2 patients on 6-MP. In view of these findings it is recommended that thyroid function be evaluated in patients on 6-MP. (Endo-crinology 94: 1583, 1974)