Serum concentrations of total and free thyroid hormones in moderately obese women during a six-week slimming cure.

Successive determinations of serum total (T4) and free (FT4) thyroxine, total (T3) and free (FT3) 3,5,3'-triiodothyronine and total 3,3',5'-triiodothyronine (rT3) concentrations were performed by radioimmunoassay in 17 moderately obese women during a 6-week slimming cure. The 2.09 MJ/d (500 kcal/d) diet (50 per cent protein, 25 per cent carbohydrate and 25 per cent lipid) was given as three daily meals of general foods. During the cure, the body mass index (BMI) decreased by about 10 per cent, with a gradual decrease in the rate of weight loss. Moreover, the patients with the highest basal overweight had the largest subsequent weight loss. A decrease of about 15 per cent in serum T4 was observed from the second week to the end of the diet period. Inversely, serum FT4 increased transiently on day 3, then returned to the baseline. The fall in serum T3 and FT3 was obvious on day 3 and an approximate 20 per cent decrease in the two hormonal forms remained from the second to the last week of diet. Serum rT3 increased by 30 per cent during the first week, then returned to the baseline on the third week. The largest decreases in serum T4, T3 and FT3 observed during the cure occurred in the patients with the highest corresponding basal values. These findings suggest that (i) the rapid changes of the serum thyroid hormone concentrations observed during the slimming cure may be an adaptive mechanism against a further weight loss, and (ii) these changes would be related to the initial state of the thyroid hormone metabolism.