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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Abstract
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 (T4), the protein-bound iodine (PBI), the per cent of endogenous T4 bound by the T4-binding globulin (TBG) or T4-binding prealbumin (TBPA), or the resin sponge uptake of triiodothyronine (T3). 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 T4. Such changes might have been indicative of an increased intensity of T4 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 T4 binding. Furthermore, comparable changes in free T4 and PBI did not accompany the decrease in FFA induced by the administration of insulin. Neither manipulation significantly affected the protein binding of endogenous T4 or the resin sponge uptake of T3. It is concluded that within a wide physiological range of concentration, FFA do not significantly influence the transport of T4 in the serum of euthyroid subjects. In the serum of patients with thyrotoxicosis, FFA may have a slight effect on the binding of T4, but the nature of any such effect is obscure, since parallel, rather than contrary changes in PBI and the proportion of free T4 followed alterations in FFA concentration.