Effect of physiological variations in free fatty acid concentration on the binding of thyroxine in the serum of euthyroid and thyrotoxic subjects.

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The effect of variations in the concentration of free fatty acids (FFA) on the binding of thyroid hormones in serum has been studied in 20 euthyroid subjects and 19 thyrotoxic patients. In the euthyroid group, neither the pronounced decreases in FFA induced by the oral administration of glucose or the intravenous administration of nicotinic acid, nor the marked increases in FFA which followed the administration of nicotinic acid or 2-deoxyglucose were accompanied by significant changes in the per cent of free thyroxine (T(4)), the protein-bound iodine (PBI), the per cent of endogenous T(4) bound by the T(4)-binding globulin (TBG) or T(4)-binding prealbumin (TBPA), or the resin sponge uptake of triiodothyronine (T(3)). In the thyrotoxic group, the decline in FFA concentration which followed glucose administration was accompanied by slight, but statistically significant, decreases in the PBI and both the per cent and absolute concentration of free T(4). Such changes might have been indicative of an increased intensity of T(4) binding secondary to the decrease in FFA. The serum PBI was decreased, however, a change contrary to that which would be expected to follow an increase in the intensity of T(4) binding. Furthermore, comparable changes in free T(4) and PBI did not accompany the decrease in FFA induced by the administration of insulin. Neither manipulation significantly affected the protein binding of endogenous T(4) or the resin sponge uptake of T(3). It is concluded that within a wide physiological range of concentration, FFA do not significantly influence the transport of T(4) in the serum of euthyroid subjects. In the serum of patients with thyrotoxicosis, FFA may have a slight effect on the binding of T(4), but the nature of any such effect is obscure, since parallel, rather than contrary changes in PBI and the proportion of free T(4) followed alterations in FFA concentration.