Association of thyroid hormones with resting energy expenditure and complement C3 in normal weight high body fat women

BACKGROUND: A high body fat percentage has a specific effect on activation of the hypothalamic-pituitary-thyroid axis. On the other hand, a slight change in thyroid hormones can affect metabolism and body composition as well as immune function. This study aims to examine the relationship between adiposity, thyroid hormone levels and immunity by comparing resting energy expenditure (REE), serum thyroid hormone levels and complement C3 in normal-weight high body fat (normal weight obesity) women and normal-weight normal body fat women. METHODS: In this case-control study, 40 women with normal body weight (BMI < 24.9 kg/m2) and body fat mass (FM) ≥ 30% (normal-weight obesity (NWO) group), and 30 non-obese women (BMI < 24.9 kg/m2) and FM < 30% (non-normal weight obesity (non-NWO) group) were recruited from a sport club in Tehran. Body composition was analyzed using bioimpedance analyzer. Blood samples were collected and analyzed for fasting serum concentration of thyroid hormones (including total T3 and total T4), thyroid-stimulating hormone (TSH), and serum complement C3. REE was measured by an indirect calorimetry. RESULTS: Serum T3 and T4 and also complement C3 were higher in the NWO group than in the non-NWO group. Body fat percentages had significant positive correlation with T3 (r; 0.344, P < 0.05), T4 (r; 0.294, P < 0.05), and complement C3 (r; 0.417, P < 0.05). Serum T3 and T4 were also positively correlated with C3 concentration (r; 0.417, p < 0.001) and (r; 0.349, p < 0.05); respectively, but there was no significant correlation between TSH and C3. REE was not significantly different between the two groups. REE only had a significant positive correlation with fat-free mass (r; 0.421, P < 0.001). CONCLUSION: An increase in body fat even in the presence of a normal body weight can be accompanied by the changes in thyroid function and inflammatory markers such as complement C3.