Effect of starvation on the production and metabolism of thyroxine and triiodothyronine in euthyroid obese patients.

The metabolic clearance and production rates of thyroxine (T4) and triiodothyronine (T3) were measured in 9 obese euthyroid patients prior to and during prolonged starvation. The metabolic clearance rates (MCR) and serum concentrations of T4, and, therefore, the metabolic degradation or production rates of T4 were unchanged during starvation. Serum T3 concentrations decreased strikingly during starvation, from 145 +/- 7 ng/dl (mean +/- SE) to 66 +/- 9 ng/dl (P < 0.001), while the mean MCR of T3 was unchanged, with the result that T3 degradation or production rates were markedly decreased (36.4 +/- 4.5 μg/d vs. 11.2 +/- 0.7 μg/d; P < 0.001). These findings suggest that the decrease in serum T3 concentration observed during starvation results from a decrease in the peripheral conversion of T4 to T3.