A Rise in Haemoglobin Levels May Enhance Serum Triiodothyronine (T₃) Concentrations in Prepubertal Patients with ß-Thalassaemia Major

Summary

The aim of the present study was to investigate the effects of increased haemoglobin (Hb) levels on the thyroid function in patients with β-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 hypo thy roidism. Five euthyroid thalassaemic patients (aged 8 to 12 years), as well as the hypothyroid thalassaemic girl, were pre-pubertal, 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 < 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 ? area) observed in EPT patients were similar to those in the EDPT group, but significantly higher in comparison with the normal children. In EPT patients and in the prepubertal hypothyroid girl the rise in Hb levels was associated with a significant increase in T₃ and fT₃ concentrations (T₃ by 1.24-fold, P < 0.01; fT₃ by 1.24-fold, P < 0.01), whereas no significant changes were found in EDPT subjects. In both groups of euthyroid thalassaemic patients the PRBC transfusion did not alter significantly basal T₄, fT₄ and TSH levels and the TSH responses to TRH. An oxygen-induced improvement in the peripheral mo-nodeiodinating system for T₄ (iodothyronine 5'-deiodinase type I) could explain the rise in T₃ and fT₃ concentrations shown in prepubertal thalassaemic patients. In the EDPT group this effect could be prevented by the tissue iron overload.

Key words

β-thalassaemia major - Thyroid function - TRH test - Haemoglobin levels