While the reduction in exercise tolerance in overt untreated hypothyroidism or overt hyperthyroidism is consistent with the underlying pathophysiology, the inconsistency of the study results of reduced exercise tolerance in patients with normal thyroid hormone levels in subclinical hypothyroidism and in patients successfully treated with levothyroxine is intriguing, because the impaired exercise tolerance recorded in normal thyroid concentrations is difficult to be explained pathophysiologically. The gene discussed is TG; the disease is hypothyroidism.