The aim of the present study was to investigate the effects of increased haemoglobin (Hb) levels on the thyroid function in patients with beta-thalassaemia major. Basal levels of thyroid hormones (T4, T3) and free thyroid hormones (fT4, fT3), basal TSH concentrations and the TSH responses to a TRH bolus (0.2 mg iv) were studied in ten euthyroid thalassaemic patients, aged 8 to 19 years, and in one 12 years-old thalassaemic girl with primary hypothyroidism. Five euthyroid thalassaemic patients (aged 8 to 12 years), as well as the hypothyroid thalassaemic girl, were prepubertal, whereas five euthyroid thalassaemic patients (aged 15 to 19 years) had delayed puberty. In each patient, the endocrine evaluation was carried out under conditions of low Hb levels (31 days after the last blood transfusion, mean Hb = 9.8 +/- 1.5 g/dl), and 11 days after the transfusion of 2 units packed red blood cells (PRBC). The latter increased significantly Hb concentrations in all the thalassaemic patients (mean Hb = 12.8 +/- 2.5 g/dl, P less than 0.001). Twelve normal prepubertal subjects, aged 6 to 11 years, served as the control group. Before the PRBC transfusion, basal T4, T3, fT4, fT3 and TSH concentrations were similar in euthyroid prepubertal thalassaemic patients (EPT) and in euthyroid patients with delayed puberty (EDPT), and were comparable to those in control subjects. The TSH responses to TRH (TSH peak, area and delta area) observed in EPT patients were similar to those in the EDPT group, but significantly higher in comparison with the normal children.(ABSTRACT TRUNCATED AT 250 WORDS)