Weight Loss after Therapy of Hypothyroidism Is Mainly Caused by Excretion of Excess Body Water Associated with Myxoedema

Abstract

In hypothyroidism, resting energy expenditure (REE) is reduced and weight gain is common. Physical activity contributes to the total daily energy expenditure, and changes in physical activity might contribute to hypothyroid-associated weight changes. The objective of the present study was to evaluate mechanisms involved in body weight changes associated with hypothyroidism. We conducted a 1-yr controlled follow-up study on outpatients newly diagnosed with hypothyroidism (n = 12) and a euthyroid measurement control group (n = 10). MAIN OUTCOME AND INTERVENTIONS: Changes in body mass and composition (dual-energy x-ray analysis scan), REE (indirect calorimetry), and spontaneous physical activity (pedometers and two different questionnaires) were studied before and after 12 months of L-T(4) therapy or observation (control group). TSH changed from 102 (85) to 2.2 (2.1) mU/liter mean (SD) and free T(4) from 4.5 (2.1) to 18 (3.3) pmol/liter after 1 yr of treatment. Body weight decreased from 83.7 (16.4) to 79.4 (16.0) kg (P = 0.002) due to change in the lean mass subcompartment only (P = 0.001) because fat and bone mass was virtually unchanged. Significant increase was observed in REE and in physical activity measured with questionnaires but not measured as daily steps. No significant changes were observed in the control group. L-T(4) therapy of hypothyroidism associated with significant decrease in body weight and increase in REE. Physical activity measured with questionnaires increased significantly, but not number of daily steps. Despite changes in REE and body weight, fat mass was unchanged during the study. We propose that total body energy equilibrium is maintained during treatment of hypothyroidism and that weight loss observed during such treatment is caused by excretion of excess body water associated with untreated myxoedema.