The relative influences of the putative thyroid stimulator, pregnancy state and non-thyroidal illnesses, were assessed in 52 patients with gestational trophoblastic neoplasia (GTN). In this study biochemical thyroid status was assessed with the aid of thyrotrophin (TSH) measurements by a sensitive immunoradiometric assay and measurement of total and free thyroid hormone concentrations. The reference group consisted of 63 patients with normal pregnancy. Thyroid function in the GTN patients was also compared with that in 36 female patients with Graves' disease and 34 female patients with non-thyroidal illnesses. We found the major influence on thyroid functions in Group I ('Hyperthyroid') patients with GTN to be the presence of the thyroid stimulator, while non-thyroidal illnesses played an important role in Group II ('High T4 syndrome') and III ('euthyroid') patients with GTN. Thyroid function was also stimulated in Group II patients. The influence of pregnancy was seen in all three groups of patients with GTN, who had higher TBG concentrations than non-pregnant patients. We conclude that there is a spectrum of thyroid function abnormalities in GTN, and that thyroid function in an individual patient is determined by the relative influence of the thyroid stimulator, non-thyroidal illnesses and the pregnancy.