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 (T) 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 T. Anthropometry including skinfold thickness (SFT) at the triceps and subscapularis was recorded. Patients underwent testing for fasting plasma glucose, creatinine, serum insulin, T, 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 T 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 T.

Conclusion: LBM decreases significantly without affecting FM after correction of hypothyroidism. Insulin resistance was not influenced by T treatment.