1. Body temperature, blood glucose, lactate, plasma free fatty acid and noradrenaline responses to exercise were studied in hyperthyroid patients during 30 min exercise on a bicycle ergometer at a work load producing a heart rate of approximately 135–140 beats/min. The results were compared with those obtained in healthy subjects exercising at work loads comparable in terms of oxygen uptake.

2. External auditory canal temperature at rest was 0·5°C higher in hyperthyroid patients than in control subjects but the exercise-induced increases in body temperature were similar in both groups.

3. Pre-exercise blood glucose concentration was higher in the thyrotoxic patients than in normal subjects. During exercise blood glucose decreased markedly in the patients whereas in the control subjects the exercise-induced changes in blood glucose were insignificant.

4. There was no difference between the groups in the pre-exercise blood lactate and plasma free fatty acid concentrations. Significantly greater increases in blood lactate and plasma free fatty acids were found during exercise in hyperthyroid patients than in the control subjects.

5. Resting plasma noradrenaline concentration in the hyperthyroid patients did not differ from that found in normal subjects, but the exercise-induced increases in plasma noradrenaline were greater in the patients than in the control group.

6. On the basis of heart rate, as well as of body temperature and blood lactate differences between the hyperthyroid patients and healthy control subjects, it appears that the patients exercising at similar oxygen uptake as normal subjects were under greater physiological strain.

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