Levothyroxine medication is associated with adiposity independent of TSH

In euthyroidism, higher TSH levels are weakly associated with increased BMI. Furthermore, a considerable number of patients complain of weight gain during thyroid hormone replacement after thyroidectomy. We therefore investigated the association between levothyroxine medication and BMI in a large cross-sectional study group.

We included euthyroid participants from the MeSyBePo study group (TSH between 0.3 and 4.5 μU/ml) that did not take thyreostatic drugs. Linear regression analyses were performed to address the association between levothyroxine medication and obesity. Additionally, pairs matched by sex, age and TSH but discordant in levothyroxine medication were compared.

1663 subjects (569 males) were eligible for inclusion. 151 participants were taking levothyroxine. Adjusted for sex and age both TSH (standardised beta 0.1, p<0.001) and levothyroxine medication (standardised beta 0.05, p=0.03) were significantly associated with BMI. There was no significant interaction between TSH and levothyroxine medication with respect to BMI. Further adjustment for smoking and the restriction to those subjects with normal glucose metabolism (947 participants (314 males, 82 on levothyroxine medication) did not alter the result. In matched pair analysis (133 pairs), BMI was significantly increased in subjects taking levothyroxine compared to controls.

Independently from TSH, levothyroxine medication was associated with a higher BMI. The mechanisms, however, responsible for this association need to be elucidated.