O₂ Uptake in hyperthyroidism during constant work rate and incremental exercise

To investigate the effect of hyperthyroidism on the pattern and time course of O₂ uptake ( {Mathematical expression}O₂) following the transition from rest to exercise, six patients and six healthy subjects performed cycle exercise at an average work rate (WR) of 18 and 20 W respectively. Cardiorespiratory variables were measured breath-by-breath. The patients also performed a progressively increasing WR test (1-min increments) to the limit of tolerance. Two patients repeated the studies when euthyroid. Resting and exercise steady-state (SS) {Mathematical expression}O₂ (ml·kg^-1·min^-1) were higher in the patients than control (5.8, SD 0.9 vs 4.0, SD 0.3 and 12.1, SD 1.5 vs 10.2, SD 1.0 respectively). The increase in {Mathematical expression}O₂ during the first 20 s exercise (phase I) was lower in the patients (mean 89 ml·min^-, SD 30) compared to the control (265 ml·min^-1, SD 90), while the difference in half time of the subsequent (phase 11) increase to the SS {Mathematical expression}O₂ (patient 26 s, SD 8; controls 17 s, SD 8) were not significant (P = 0.06). The OZ cost per WR increment (Δ {Mathematical expression}O₂/ΔWR) in ml·min^-1·^-1, measured during the incremental period (mean 10.9; range 8.3-12.2), was always within two standard deviations of the normal value (10.3, SD 1). In the two patients who repeated the tests, both the increment of {Mathematical expression}O₂ from rest to SS during constant WR exercise and the Δ {Mathematical expression}O₂/ΔWRs during the progressive exercise were higher in the hyperthyroid state than during the euthyroid state. While both resting and exercise {Mathematical expression}O₂ are increased in the hyperthyroid patients, the O₂ cost of a given increment of WR is within the normal range. However, a small reduction in the {Mathematical expression}O₂ requirement to perform exercise following treatment of the hyperthyroid state suggests a subtle change O₂ cost of muscle work in this disease.