Thyroid hormones (TH) are potent modulators of adaptive thermogenesis and can potentially contribute to development of obesity. The decrease of T(3) in association with reduction of calorie intake is centrally regulated via decreases in leptin and melanocortin concentrations and peripherally via a decrease in deiodinase activity, all aimed at protein and energy sparing. The use of TH in the treatment of obesity is hardly justified except in cases of elevated thyrotropin (TSH) with low/normal T(3) and T(4) and/or a low T(3) or T'(3)/T(4) or a high TSH/T(3) ratio. TH treatment with small doses of T(3) can also be exceptionally applied in obese patients resistant to dietary therapy who are taking beta-adrenergic blockers or with obesity developed after cessation of cigarette smoking and with hyperlipidemia and a concomitant high thryrotropin/T(3) ratio. Supplementation with Se(2+) and Zn(2+) may be tried along with more severe calorie restriction to prevent decline of T(3).