Thyroid function is more strongly associated with body impedance than anthropometry in healthy subjects

Since fat-free tissues are responsible for 95% of basal energy expenditure, fat-free mass is expected to be a better determinant of thyroid size and function than anthropometry. We tested the hypothesis that fat-free tissues as qualitatively determined by body resistance (R) at 50 kHz are more strongly associated with TSH than anthropometric indicators in healthy subjects. A number of 78 euthyroid adults of both sexes were consecutively studied. R was the best single predictor of TSH (R2(adj)=0.65, p<0.0001). It explained 36% more variance than bw (R2(adj)=0.29, p<0.0001), the most accurate anthropometric predictor. Sex had no effect on the relationship between TSH, bioelectrical impedance analysis and anthropometry. After the contribution of R to TSH was taken into account, anthropometric indicators were not able to explain any additional part of TSH variance. We conclude that in healthy subjects, bioelectrical resistance is a better indicator of thyroid function than anthropometry, probably because of its more direct relationship with fat-free tissues. Further studies are needed to test whether this relationship holds in under- and over-weight subjects.