Purpose: Thyroid hormones (THs) are important mediators of brown adipose tissue (BAT) differentiation. However, the association of TH concentrations with human BAT is unclear. The present work examines the associations between circulating thyroid-stimulating hormone (TSH) and THs concentrations (i.e. free tri-iodothyronine, FT3, and free thyroxine, FT4), under thermoneutral [22-23ºC] and cold-induced conditions, and BAT volume, 18F-fluorodeoxyglucose (18F-FDG) uptake and mean radiodensity.
Methods: A total of 106 young healthy, euthyroid adults (34 men/72 women; 22.0 ± 2.1 years old; 24.9 ± 4.6 kg/m2) participated in this cross-sectional study. BAT volume, 18F-FDG uptake and mean radiodensity were assessed after 2 h of personalized (i.e., contemplating each individual's shivering threshold) cold exposure via positron emission tomography/computerized tomography (PET/CT) static scanning. TSH and THs levels were determined before (thermoneutral) and 1 h after the cold exposure.
Results: Cold exposure increased circulating FT4 (P=0.038) and reduced of TSH levels (P≤0.001). Conversely, the FT3 serum concentration was not modified by cold exposure (P=0.435). No associations were found between the TSH and THs thermoneutral (all P>0.289) or cold-induced levels (all P>0.067) and BAT volume,18F-FDG uptake and mean radiodensity. These findings were independent of sex and body mass index.
Conclusions: Thyroid function is modulated by cold exposure, yet it is not associated with BAT volume or glucose metabolism assessed after 2h of cold exposure in young healthy, euthyroid adults.