The metabolic fate of 11β-hydroxy-Δ4-androstene-3,17-dione has been studied in man under different levels of thyroid hormone. Several patients were studied at 2 levels of thyroid function. The metabolites derived in hyperthyroid subjects were qualitatively and quantitatively the same as those obtained in euthyroid subjects with 3α,11β-dihydroxyandrostan-17-one as the principal product. In hypothyroid subjects there was a decrease in the production of 3α,11β-dihydroxy-5β-androstan-17-one with a concomitant rise in the formation of 3,11β-dihydroxy-Δ4-androsten-17-ones. The amounts of the 5β-steroids produced were essentially the same in euthyroid, hyperthyroid and myxedematous subjects. It was concluded that the 5α-reductase and Δ4-3-hydroxysteroid dehydrogenase associated with 11β-hydroxy-Δ4-androstene-3,17-dione are responsive to low levels of thyroid hormone.