Lean body mass is a predictor of the daily requirement for thyroid hormone in older men and women.

Levothyroxine (T4) replacement need in adults with primary hypothyroidism has recently been reported to fall with age. Previous studies have demonstrated that the resting metabolic rate falls with age in euthyroid adults and that this fall is proportional to a reduction in lean body mass (LBM). Since LBM is correlated also with 24-hour energy expenditure, this study examined the possibility that LBM might be an accurate predictor of T4 requirement. Seventy-five hypothyroid adults receiving full replacement therapy, ranging in age from 24 to 88 years, were studied retrospectively. Lean body mass was found to be a better predictor of T4 requirement than age or weight for the entire group as well as for subgroups of men and women 51 years old and older. The age-related reduction in LBM may be responsible for the reported decrease in the rate of fractional thyroxine degradation with age.