THYROID STATUS AND EXERCISE TOLERANCE - CARDIOVASCULAR AND METABOLIC CONSIDERATIONS

Both hypo- and hyperthyroidism are characterised by exercise intoleran ce. In hypothyroidism, inadequate cardiovascular support appears to be the principal factor involved. Insufficient skeletal muscle blood flo w compromises exercise capacity via reduced oxygen delivery, and endur ance through decreased delivery of blood-borne substrates. The latter effect results in increased dependence on intramuscular glycogen. Addi tionally, decreased mobilisation of free fatty acids from adipose tiss ue and, consequently, lower plasma free fatty acid levels compound the problem of reduced lipid delivery to active skeletal muscle in the hy pothyroid state. In contrast, cardiovascular support is enhanced in hy perthyroidism, implicating other factors in exercise intolerance. Grea ter reliance on muscle glycogen appears to be the primary reason for d ecreased endurance. Biochemical changes with hyperthyroidism that woul d favour enhanced flux through glycolysis may account for this depende nce on glycogen. Deviations from normal thyroid function, and the ensu ing exercise intolerance, require appropriate medical therapy to attai n euthyroid status.