Thyroid function is associated with body mass index and fasting plasma glucose in Thai euthyroid population

Several population-based studies found the associations between body mass index and thyroid function within the normal range. Furthermore, these thyroid functions are related with insulin resistance and plasma glucose levels. This study aimed to investigate the associations between thyroid functions and metabolic parameters in Thai euthyroid population.

Participants from the Thai National Thai Health Examination Survey were randomly measured for TSH, FT4, anti-thyroperoxidase, and anti-thyroglobulin. Euthyroidism was defined by TSH 0.27-4.20 mIU/L and FT4 0.93-1.71 ng/dL.

A total of 2242 euthyroid participants were included. Fifty-one percent were female. Mean age, fasting plasma glucose, and body mass index were 55 ± 21 years, 93 ± 29 mg/dL, and 23.4 ± 4.6 kg/m², respectively. Multivariate regression analysis after age and sex adjustment showed a negative association of serum FT4 with body mass index (β = -0.070, p = 0.001) and the relationship was still significant after subjects with positive anti-thyroperoxidase were excluded (β = -0.068, p = 0.003). In contrast, serum TSH was positively associated with body mass index (β = 0.052, p = 0.012). Moreover, serum FT4 was positively associated with fasting plasma glucose levels (β = 0.097, p < 0.001).

Small variations of serum TSH and FT4 within the reference range may contribute to the differences in metabolic indexes such as body mass index and fasting plasma glucose.