Correction of Hypothyroidism Leads to Change in Lean Body Mass without Altering Insulin Resistance

Background: Hypothyroidism is associated with insulin resistance, dyslipidemia, and abnormal body composition. This study assessed changes in body composition and insulin resistance after thyroxine (T₄) replacement in overt hypothyroidism. Methods: In this prospective longitudinal study carried out in a tertiary care center, adult nondiabetic patients with overt hypothyroidism were rendered euthyroid on T₄. Anthropometry including skinfold thickness (SFT) at the triceps and subscapularis was recorded. Patients underwent testing for fasting plasma glucose, creatinine, serum insulin, T₄, thyrotropin (TSH) and body composition analysis by dual-energy X-ray absorptiometry (DEXA) both before and at 2 months after restoration to the euthyroid state. Results: Twenty-seven patients (20 female and 7 male) aged 35.3 ± 11.0 years (min-max: 17-59 years) with overt hypothyroidism were recruited. Serum T₄ at the time of recruitment was 48.9 ± 24.6 nmol/l (normal range = 64.4-142 nmol/l). All patients had TSH ≥50 µIU/l. Following treatment, there was a mean body weight reduction of 1.7 kg (p = 0.01). Waist circumference as well as triceps and subscapularis SFT decreased significantly (p < 0.001). There was no change in fat mass (FM), percentage of fat (%FM) or bone mineral content in any of the specified regions or in the body as a whole. In contrast, mean lean body mass (LBM) decreased significantly by 0.8 kg (p < 0.01) in the trunk and 1.3 kg (p < 0.01) in the whole body. Insulin resistance and level of glycemia were not affected by treatment with T₄. Conclusion: LBM decreases significantly without affecting FM after correction of hypothyroidism. Insulin resistance was not influenced by T₄ treatment.