Eur J Clin Invest 2011; 41 (11): 1202–1209
Background Different studies, mostly cross-sectional, have found an association between low levels of thyroid hormones, even within the normal range, and a greater body mass index. The aim of this study was to determine the association between thyroid function and the risk for obesity.
Materials and methods In this population-based prospective study, measurements were made of anthropometric parameters, thyroid hormone function and urinary iodine in a cohort of the Pizarra Study (n = 937), and repeated 6 years later (n = 784). At the second point, measurements were also made of leptin and adiponectin.
Results Among the persons who were not obese at the start of the study, the odds ratio (OR) of becoming obese for those in the fourth quartile (Q4) for free triiodothyronine (FT3) (versus those in Q1) was 2·94 (1·46–5·90) (P = 0·005). The OR of becoming obese in persons in Q4 of FT4 (versus those in Q1) was 3·06 (1·23–7·43) (P = 0·01). Those persons in Q4 of weight gain had a higher FT3 at the 6-year follow-up than those whose weight gain was in Q1 (P < 0·001). Leptin correlated with thyrotropin (β = 0·58, P = 0·001) and the FT4 (β = −1·12, P = 0·005). Adiponectin correlated with FT3 (r = −0·24, P < 0·001). The urinary iodine correlated negatively with both the BMI (β = −0·08, P = 0·01) and the increase in weight (β = −0·08, P = 0·04).
Conclusions The changes in the thyroid hormones could be the consequence, rather than the cause, of the increase in weight. The same pathophysiological mechanisms that induce obesity might also be modifying the thyroid hormone pattern.