Studies on thyroid function in obesity yielded inconsistent results; high thyroid-stimulating hormone (TSH) levels were generally shown; high free triiodothyronine (fT)-3 or fT4 levels were described in some, but not in other studies. After weight loss, TSH and thyroid hormones have been described to either increase or decrease. Our aim was to describe TSH, fT3, and fT4 in obese subjects with normal thyroid function before and after durable and significant weight loss, obtained through laparoscopic gastric banding (LAGB), in comparison with nonobese subjects. TSH, fT3, fT4, and fT3/fT4 ratio (an index of D1 and D2 deiodinase activity), were evaluated in 99 healthy controls and in 258 obese subjects, at baseline and 6 months, 1 year, and 2 years after LAGB, together with indexes of glucose (glucose, insulin, homeostasis model assessment of insulin resistance index) and lipid (triglycerides, total and high-density lipoprotein-cholesterol) metabolism, and anthropometric measures (BMI and waist circumference). Under basal conditions, TSH, fT3, and fT4 were all in the normal range, but higher in obese than in nonobese subjects, and fT3/fT4 ratio was normal; with weight loss, fT3 and fT3/fT4 ratio decreased in obese subjects, while fT4 increased and TSH remained steady; all values were again within the normal range. Albumin and cholesterol levels remained steady, while triglycerides, insulin, and homeostasis model assessment of insulin resistance decreased, and high-density lipoprotein-cholesterol increased. These changes, however, do not modify TSH, letting us to hypothesize that the changes are due to a decrease of D1 and D2 deiodinase activities.