To evaluate the effect of thyroid function on the in vivo cortisol ⇌ cortisone (F ⇌ E) equilibrium, double-labeled cortisol tracer techniques were used to measure separately the rates of the F → E and E → F reactions in 4 euthyroid, 2 hypothyroid, and 2 hyperthyroid subjects. The rate of the F → E reaction was calculated from the appearance rate of 3H in body water after the i.v. injection of [11α-3H]-cortisol; the rate of the E → F reaction was calculated from the difference in the plasma turn-over rates of [11α-3H]-cortisol and [4-14C]-cortisol after simultaneous i.v. injection of both tracers; the F ⇌ E set-point was calculated by dividing the F → E rate by the E → F rate. In euthyroid subjects the F → E reaction rate averaged 1.6%/min, the E → F reaction rate averaged 1.0%/min, and the F ⇌ E set-point averaged 1.6. In hyperthyroid subjects the two reaction rates were supranormal: the F → E rate averaged ≥ 3.9%/min and the E → F rate averaged 1.54%/min; since the increase in the F → E rate was proportionally greater, the F ⇌ E set-point (average ≥ 2.6) was displaced towards cortisone. In hypothyroid subjects both reaction rates were equally slowed, to an average of 1.0%/min (F → E) and 0.68%/min (E → F); the F ⇌ E set-point averaged 1.65, the same as in euthyroid subjects. Displacement of the F ⇌ E equilibrium towards cortisone in hyperthyroid subjects appears to account for their elevated urinary THE/THF ratios, but the normal F ⇌ E set-point in hypothyroid subjects makes it necessary to invoke a different mechanism for their depressed THE/THF ratio; it is suggested that increased conversion of THE to cortoic acids may be the responsible factor.