Endocrine Abstracts

Background: Thyroid hormones have been shown to be necessary for proper function of brown adipose tissue (BAT) and cold induced thermogenesis (CIT) in rodent models. In humans hyperthyroidism has been demonstrated to activate BAT and increase thermogenesis. However, the data on CIT in humans in the hypothyroid vs. the euthyroid state are limited and conflicting.

Methods: This was a prospective observational study. We recruited 40 patients presenting to a University hospital out-patient endocrine clinic with subclinical or overt hypothyroidism, and in whom thyroid hormone replacement was planned. Currently, 24 patients completed the study. Thermogenesis was measured by indirect calorimetry during warm conditions and after a mild cold stimulus of 90 min, both during the hypothyroid state and after at least 3 months of sufficient thyroid hormone replacement. CIT was determined as the difference between resting energy expenditure (REE) during cold (REEcold) and warm (REEwarm) conditions. Additionally, skin temperature was measured using wireless probes in one-minute intervals in the supraclavicular region adjacent to the major human BAT depot. The primary endpoint was CIT in the hypothyroid vs euthyroid state. Data were analysed by non-parametric tests and are given as mean±S.D.

Results: CIT increased from 73(±103) kcal/d during the hypothyroid state (CIThypo) to 160 (±143) kcal/d in the euthyroid state (CITeu), P=0.0007. REEwarm was 1324 (±120) kcal/d during hypothyroidism and rose slightly to 1395(±140) kcal/d, P=0.0137. REEcold rose profoundly from 1396(±157) kcal/d to 1555(±178) kcal/d, P=0.0001. While the inter-individual variability of CIT was large, the correlation between CIThypo and CITeu was moderate, r=0.657, P=0.001. The change in supraclavicular skin temperature during cold exposure correlated with CIT in the euthyroid (r=0.389, P=0.067), but not in the hypothyroid state (r=0.041, P=0.853).

Conclusion: Cold induced thermogenesis is blunted in hypothyroid patients and increases markedly after restoration of euthyroidism. The difference in CIT is larger than the difference in resting energy expenditure during warm conditions.