Relationship between thyroid dysfunction and body weight: a not so evident paradigm

PURPOSE: Hypothyroidism has traditionally been associated with obesity, whereas hyperthyroidism has been linked to being underweight. However, very few studies have assessed these associations. The aim of this work is to evaluate the association between thyroid dysfunction and body mass index (BMI) at baseline and after normalization of the hormone levels. PATIENTS AND METHODS: A retrospective, observational study of a cohort of otherwise healthy patients that were referred for evaluation of thyroid dysfunction to the Endocrine Department of Pontevedra University Complex Hospital, Spain was conducted. We collected data of BMI and thyroid hormone levels before treatment and after normalization of thyroid function within a follow-up period of 12 months. RESULTS: A total of 330 patients were initially selected for the study. In order to exclude variables that for any reason could influence on BMI, 235 were excluded for further studies. Another 61 patients were also excluded because incomplete data on their medical records, failure to achieve euthyroidism, or lost to follow-up. Therefore, the eligible final study group consisted of 34 patients (17 with hypothyroidism and 17 with hyperthyroidism). No differences were observed in mean baseline BMI between hypo and hyperthyroid patients (27.07±3.22 vs 26.39±4.44, p=0.609). Overweight or obesity was observed in 76.5% and 58.8% of hypothyroid and hyperthyroid patients, respectively (p=0.23). After normalization of thyroid function, the weight of hypothyroid patients decreased from 70.93±10.06 kg to 68.68±10.14 (p=0.000), while the weight of hyperthyroid patients increased from 65.45±11.64 kg to 68.37±12.80 (p=0.000). Their mean BMI was 26.22±3.36 and 27.57±4.98 (p=0.361) for hypo- and hyperthyroid patients, respectively. 58.8% and 64.7% patients remained in the overweight/obesity range in each group (p=0.72). CONCLUSION: Untreated thyroid dysfunction is not associated with BMI. Normalization of thyroid levels significantly changed the weight of patients, but remaining most patients within overweight ranges.