HUMAN LYMPHOCYTE BINDING AND DEIODINATION OF THYROID HORMONES IN RELATION TO THYROID FUNCTION

Abstract

The human lymphocyte has been investigated regarding its function as a thyroid hormone target cell. Binding and deiodination of the thyroid hormones were determined after simultaneous incubation of ¹³¹I-labelled L-thyroxine (¹³¹I-T₄) and ¹²⁵I-labelled L-triiodothyronine (¹²⁵I-T₃) with lymphocytes from healthy subjects, from hyperthyroid and primary hypothyroid patients before and after treatment. The mean percentages of binding, 8.0 ± 0.5 (mean ± sem) for ¹³¹I-T₄, and 9.7 ± 0.4 for ¹²⁵I-T₃ in the control group, were increased in the hyperthyroids to 10.1 ± 0.4 and 12.7 ± 0.6 respectively, and in the hypothyroids to 10.9 ± 0.7 and 12.8 ± 0.6. All elevated values returned to normal with successful treatment. The mean percentage of deiodination, 12.0 ± 1.7 for ¹³¹I-T₄, and 6.5 ± 0.9 for ¹²⁵I-T₃ in the control group, showed a threefold increase in the hyperthyroid patients, to 35.9 ± 3.2 and 20.2 ± 1.9 respectively and remained unaltered in the hypothyroid patients. The values of successfully treated hyperthyroid patients were normal and those of the treated hypothyroid patients below normal. Human TSH added to the incubation medium stimulated the binding of both hormones, without influencing deiodination. Thus TSH might be active at the peripheral cellular level. This could contribute to the explanation of the increased binding by lymphocytes from primary hypothyroid patients with high serum concentrations of TSH. A preliminary analysis of the binding characteristics revealed an equilibrium affinity constant of 1.03·10¹⁰ m⁻¹ for T₃, and of 3.97· 10⁹ m⁻¹ for T₄, with corresponding total numbers of binding sites of 1500 and 2000 per cell. It is concluded that, since lymphocyte activities closely reflect the thyroid function, these cells are well suited for studies on the peripheral fate of thyroid hormones and their cellular receptors.