THE EFFECT OF CALORIE RESTRICTION ON SERUM THYROID HORMONE BINDING PROTEINS AND FREE HORMONE IN OBESE PATIENTS

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Abstract
In 10 obese euthyroid subjects the concentration of thyroxine (T4), triiodothyronine (T3) and reverse [r] T3 were assayed in serum and of T4 and T3 in urine before and after 2 wk 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 6 subjects the concentrations of serum T4 binding globulin (TBG), T4 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 percent free T4. Calorie restriction has a selective effect on the concentrations of TBPA and to a lesser extent TBG. Altered peripheral monodeiodination of T4 also occurs. Decreased urinary excretion of T4 despite a calculated increase of free T4 in serum, suggests intrarenal metabolic adjustment, the mechanism of which awaits elucidation.