Association between Thyroid Function and Body Mass Index: A 10-Year Follow-Up

Abstract

Background/Aims: We aimed to evaluate the association between change in thyroid function tests within the euthyroid range and body mass index (BMI) in persons with normal weight at baseline. Methods: This study investigated 1,100 normal-weight euthyroid persons in a population-based cohort study, Tehran Thyroid Study. BMI was calculated and serum concentrations of thyrotropin (TSH) and free T4 (FT4) were assayed at baseline and after 10 years of follow-up. We evaluated the relationship between thyroid and obesity based on 2 definitions for outcome: (1) a binary outcome as BMI <25 or ≥25 kg/m², and (2) a multinomial outcome as normal BMI, overweight, and obese. Results: A total of 569 women and 531 men, aged 36.3 ± 13.5 years, were included. Modified Poisson regression analysis for binary outcome, after adjustment for age, sex, smoking, and anti-thyroid peroxidase antibody status, revealed a negative association between delta serum FT4 and follow-up BMI (relative risk 0.55 [95% CI 0.37-0.80]) without any significant association between change in serum TSH and follow-up BMI. However, in multinomial logistic regression analysis, we found no relationship between delta serum FT4 or TSH and follow-up BMI categories, for either overweight or obese vs. normal-weight participants. Conclusions: In normal-weight euthyroid individuals, changes in serum concentrations of FT4, but not TSH, may contribute to change in body weight.

Association between thyroid hormone levels, body composition and insulin resistance in euthyroid subjects with normal thyroid ultrasound: The Kangbuk Samsung Health Study

“…Thyroid hormones play crucial roles in the control of energy homoeostasis and thermogenesis and can therefore influence body composition . Hypothyroidism is associated with weight gain, whereas hyperthyroidism is associated with increased metabolic rate and weight loss .…”

“…In contrast, the changes in body composition might influence thyroid hormone levels. Previous studies have evaluated the associations between thyroid hormone levels and anthropometric markers . Serum thyrotropin (thyroid‐stimulating hormone, TSH), triiodothyronine (T3) and free T3 (FT3) levels were positively associated with body mass index (BMI) and waist circumference (WC) in euthyroid subjects .…”

“…3 Thyroid hormones play crucial roles in the control of energy homoeostasis and thermogenesis and can therefore influence body composition. [4][5][6] Hypothyroidism is associated with weight gain, whereas hyperthyroidism is associated with increased metabolic rate and weight loss. 7 Changes in thyroid function may contribute to the development of metabolic complications, even when thyroid hormone levels are within the normal range.…”

“…Previous studies have evaluated the associations between thyroid hormone levels and anthropometric markers. 4,5,[9][10][11][12][13][14][15][16][17][18][19][20] Serum thyrotropin (thyroid-stimulating hormone, TSH), triiodothyronine (T3) and free T3 (FT3) levels were positively associated with body mass index (BMI) and waist circumference (WC) in euthyroid subjects. 11,12,14,15 However, the associations of serum thyroxine (T4) and free T4 (FT4) levels with obesity are controversial.…”

“…Many previous studies have evaluated the associations between thyroid hormone levels and BMI, WC or waist-to-hip ratio, 4,11,12,[14][15][16][17] because these are widely used metrics for obesity. However, few studies have used body composition such as body fat mass and body fat percentage (BFP) as surrogate parameters.…”