Bioelectrical impedance assessment of body composition in thyroid disease

To assess the metabolic effects of thyroid disease, body composition was determined by bioimpedance analysis (BIA) in 72 patients with untreated hyperthyroidism (mean age 48.7 +/- 1.9 years) and 26 patients with untreated hypothyroidism (63.8 +/- 3.4 years). Bioelectrical whole body resistance (R) and reactance (Xc) were used for computerized calculation of lean body mass (LBM), body cell mass (BCM), extracellular mass (ECM) and body fat (BF). Compared with age- and sex-matched healthy controls the most sensitive parameter indicating excess thyroid hormone was the ECM/BCM ratio which was markedly elevated in all hyperthyroid subjects. ECM/BCM alteration resulted from marked depletion of BCM with concomitant expansion of ECM. BCM change is thought to be predominantly due to a loss of muscle mass while ECM rise may reflect an increase in extracellular fluids. In contrast, hypothyroidism was characterized by an increase in BF besides a relatively unaffected LBM component. Serum parameters of thyroid function (tri-iodothyronine (T3), free thyroxine, TSH) did not correlate with the determinants of body composition except for a slight inverse relationship between the phase angle (Xc/Rx180 degrees/pi) and T3 concentration in Graves' disease patients. We conclude that hyperthyroidism is primarily accompanied by quantitative as well as qualitative changes in the lean body while considerable fat increase is the most important feature of hypothyroidism. Severity of body composition derangement cannot be predicted from the degree of thyroid dysfunction. BIA could become a useful tool which allows objective determination of even subtle metabolic manifestations of thyroid disease and should, therefore, complement conventional clinical and biochemical assessment.