Comparison of measurements of body composition by total body potassium, bioimpedance analysis, and dual-energy X-ray absorptiometry in hypopituitary adults before and during growth hormone treatment

We compared fat-free mass (FFM) and percentage body fat mass (BFM) values derived from total body potassium (TBK), bioelectrical impedance analysis (BIA), and dual-energy X-ray absorptiometry (DXA) in hypopituitary adults before and after 6 mo treatment with growth hormone. Before growth hormone treatment, FFM values from the three methods correlated strongly. FFM values from TBK were lower than FFM values derived from BIA and DXA (x̄ ± SD: 53.7 ± 14.3 compared with 49.1 ± 9.2 kg, P < 0.0001; DXA compared with TBK: 54.7 ± 16.4 and 49.2 ± 9.7 kg, P < 0.0002). BFM values from TBK were significantly higher than the BIA-derived (P < 0.002) but not different from the DXA-derived values. There was no difference in FFM and BFM values derived from DXA and BIA methods. The differences between BIA and TBK methods and between DXA and TBK methods were observed in the obese but not in the nonobese subjects. The increase in FFM derived from BIA with growth hormone was greater than that derived from TBK ([median(range)]; BIA: +5.2(-0.1, +13.8) compared with TBK: +0.9(-4.8, +8.6) kg, P < 0.001, but the changes with placebo were not different. The changes in FFM and BFM derived from DXA with growth hormone and placebo were not significantly different from those derived by using TBK or BIA. We conclude that FFM and BFM values derived from TBK, BIA, and DXA correlate highly and that TBK-derived values for FFM are lower than those derived from BIA and DXA in obese patients.