Serum concentrations of total T4, T3, reverse T3 and free T4, T3 in moderately obese patients.

Serum total thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3), 3,3',5'-triiodo-L-thyronine (reverse T3 or rT3) and free thyroxine (FT4), free 3,5,3'-triiodo-L-thyronine (FT3) concentrations were measured by radioimmunoassay in 52 moderately obese patients and 32 age-matched control subjects with constant body weight and no dietary restriction. The comparisons of iodothyronine concentrations in the obese and control subjects gave the following results: serum T4 and FT4 concentrations were not different in the two samples (mean +/- s.e. T4: 105.8 +/- 3.0 vs 109.4 +/- 4.0 nmol/l, FT4: 10.9 +/- 0.4 vs 10.8 +/- 0.8 pmol/l). A slight but non-significant decrease in serum rT3 concentrations was observed in the obese patients (0.29 +/- 0.02 vs 0.32 +/- 0.02 nmol/l). A slight (10 per cent) but highly significant (P less than 0.01) decrease was observed in the obese patients for serum T3 (2.20 +/- 0.06 vs 2.44 +/- 0.05 nmol/l) and FT3 (6.0 +/- 0.2 vs 6.9 +/- 0.2 pmol/l). Serum FT4 and T4 on the one hand and serum FT3 and T3 on the other were positively correlated both in the obese and control subjects. No correlation was found between serum T3 and weight expressed as body mass index. The results suggest that the most important factor responsible for the decrease in the serum total and free T3 concentrations is the nutrient intake.