In recent years transient primary hypothyroidism has reported with increasing frequency. Physicians are often unsure whether withdrawal of thyroid hormone to identify the transient hypothyroidism is indicated and cost-effective and in which patients this should be done. To study these questions, thyroid hormone therapy was withdrawn from 63 patients with proven primary hypothyroidism at 6 months and again at 1 and 3 years to determine if there was recovery of thyroid function. Of the 49 patients with primary hypothyroidism (PH) that was not attributable to such causes as drug therapy, surgery, iodine-131 therapy, or silent or subacute thyroiditis, only two patients recovered thyroid function. In the other 14 patients, hypothyroidism developed within 6 months postpartum. Nine of these 14 recovered thyroid function. Therefore, it appears that when PH is not related to certain specific causes or states, it is likely to be permanent. Furthermore, withdrawal of thyroid hormone therapy to assess recovery of thyroid function is unnecessary and not cost-effective.