We examined 103 euthyroid men and women within a wide range of body weights and ages. Fat free mass (FFM) and body fat (BF) were determined with the total body potassium technique, basal metabolic rate (BMR) by indirect calorimetry and serum concentrations of thyroid hormones (free and total T3 and T4) and the aminoterminal propeptide of collagen III (pIIIp) by immunoassays. BMR was positively related to FFM, BF, total T3, the free T3/free T4 ratio and pIIIp, and negatively to free T4 (men) and to the ratios free T4/total T4 and free T3/total T3. pIIIp was as strongly related to BMR as to total T3. It is suggested that pIIIp may serve as an indicator of peripheral energy expenditure. The negative relationship between BMR and free T4 was unexpected and different to the situation in hypo- and hyperthyreosis where BMR and thyroid hormone are positively related. Our hypothesis is that euthyroid subjects with low serum free thyroid hormone concentrations and comparatively high BMR may have high intracellular thyroid hormone concentrations.