A patient with isolated thyrotropin (TSH) deficiency was studied. Prior to treatment with thyroid hormone, administration of thyrotropin releasing hormone (TRH) produced no increment in serum TSH and a normal increase in plasma prolactin (PRL). In order to explore whether physiologic increases in serum TSH might be occurring below the limits of detectability of TSH by radioimmunoassay, a double isotope technique of assessing thyroidal secretion secondary to release of TSH was employed. The patient was restudied seven months later, after discontinuing thyroid hormone replacement therapy for two months, and on this occasion repeat TRH administration produced small increments in serum TSH. After administration of 125I and 131I-T4 to assess thyroid hormone secretion, TRH was infused continuously for 6 h. Small increases in serum TSH were again observed, along with significant increases in PG125I/PB131I and urinary 125I/131I, reflecting increased thyroidal iodine secretion, although serum T3 and T4 did not change. These studies indicate that: 1) isolated TSH deficiency need not be complete and may be associated with detectable levels of immunoassayable TSH; 2) the TSH released possesses in vivo biological activity; and 3) therapy with thyroid hormone may have facilitated TSH release.