Respiratory adaptation to hyperthyroidism

¹ Department of Physiology, Dartmouth Medical School, Hanover, New Hampshire

In order to test the general postulate that increases in whole body metabolism are accompanied by net respiratory stimulation, respiratory adaptation to thyrotoxicosis was studied in clinical hyperthyroidism and in experimental hyperthyroidism in man and the rat. It was found that in contrast to other hypermetabolic states such as fever and salicylate and dinitrophenol administration, alveolar CO₂ tension rises in hyperthyroidism, reflecting a proportionately greater increase in metabolism (CO₂ production) than in alveolar ventilation. Ventilatory response curves in three human subjects with experimentally induced hyperthyroidism showed no increase in sensitivity (a/Pa_COCO2 of the respiratory center to CO₂. This finding differs from that in salicylism and dinitrophenol administration in which enhanced sensitivity is manifest. It is concluded that the respiratory stimulation resulting from increased CO₂ production is modified by factors such as changes in respiratory center sensitivity, changes in cerebral blood flow and direct stimulation of the respiratory center, so that the net respiratory adjustment cannot be predicted from a knowledge of the increase in metabolism alone.