The effects of oestrogens on the release of thyroxine iodide (T4 I) and non-T4 I by the thyroid gland of 6 normal subjects and 3 thyrotrophin (TSH)-treated patients with hypopituitarism were assessed before, during and after treatment with 25 mg of Dienoestrol (Glaxo Lab.), orally, daily, during 8 days. Each subject received an iv injection of 150 μCi of 125I followed 6 days later by an iv injection of 75 μCi of [131I]T4. Blood samples and collections of urine were obtained every 12 h. From plasma and urinary (U) radioactivity, the ratio U[125I]/ U[131I], U[125I]/PB[125I] and U[131I]/PB[131I] were obtained and served to calculate thyroidal iodine release and non-T4 I secretion by the thyroid gland. T4 I secretion was equivalent to T4 I disposal rate obtained by the semilogarithmic regression line of PB[131I]. In normal subjects, oestrogens decreased T4 I secretion from 491 ± 57 (sd) nmol/day to 451 ± 60 nmol (P < 0.025), while in the same subjects serum TSH increased with oestrogens from 3.3 ± 1.1 to 6.6 ± 2.1 mU/l (P < 0.01). The non-T4 I secretion had a mean of 585 ± 307 nmol/day during control, 324 ± 199 nmol during oestrogens (P < 0.01) and 480 ± 286 nmol after oestrogens (P < 0.05 vs oestrogens). In the TSH-treated patients oestrogens induced non-significant reductions in T4 I and non-T4 I secretion, although total iodine secretion decreased significantly (P < 0.05). The data is consonant with a partial inhibition by oestrogens of thyroid gland release.