BACKGROUND: Published results of studies of thyroid function in obesity and after weight loss have differed. METHODS: The circulating concentrations of thyroid hormones and TSH were studied in 30 consecutive, euthyroid morbidly obese patients before and after weight loss from vertical gastroplasty, with the aim to determine the relation between body weight loss and pituitary-thyroid axis function. Serum TSH, free T3 (FT3) and free T4 (FT4) were measured before operation and repeated 6 and 18 months postoperatively. RESULTS: A significant increase, but within normal levels, in FT3 value at 6 and, mainly, at 18 months after gastroplasty was observed (p = 0.002). The FT4 value was slightly increased, at the same time and serum TSH was found to, be significantly decreased (p = 0.03 and p = 0.01 respectively). The relative increase in FT4 was negatively correlated with excess body weight and Body Mass Index reduction. This correlation was only moderate with values ranging from r = 0.34 to r = 0.47. CONCLUSIONS: Although there were statistically significant differences in thyroid function tests before and after loss of weight, these were not biologically significant. The hypophyseal/thyroid axis remains always active and contributes to body weight homeostasis and its regulation.