The effect of calorie restriction on serum thyroid hormone binding proteins and free hormone in obese patients

In ten obese euthyroid subjects the concentration of thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3) were assayed in serum and of T4 and T3 in urine before and after 2 weeks of 2.1 MJ (500 Kcal) per day diet. Mean serum T4 was unchanged, while T3 decreased and rT3 increased. Urinary excretion of both T4 and T3 decreased after diet. In six subjects the concentrations of serum thyroxine binding globulin (TBG), thyroxine binding prealbumin (TBPA) and albumin were measured before and after diet. Free T4 and T3 were calculated using a formula based on measured concentration of hormones and their binding proteins. Calorie restriction resulted in a significant decrease in TBG and a greater decrease in TBPA, while albumin was unchanged. Calculation of free hormones showed a fall in absolute free T3 and rise in percentage free T3, but a rise in both absolute and per cent free T4. Our data indicate a selective effect of calorie restriction on the concentrations of TBPA and to a lesser extent TBG and confirm previous reports of altered peripheral monodeiodination of T4. Decreased urinary excretion of T4, despite a calculated increase of free T4 in serum, suggests intrarenal metabolic adjustment, the mechanism of which awaits elucidation.