Thyrotoxicosis increases right to left shunt in congenital cyanotic heart disease

We wish to report two cases of congenital cyanotic heart disease in whom coexisting thyrotoxicosis increased the right to left shunt. We review the literature on the cardiac and respiratory effects of thyroid hormone to try to explain this phenomenon. Two patients, one with Fallot's tetralogy (diagnosed at 68 years of age), the other with a ventricular septal defect, developed thyrotoxicosis which resulted in a dramatic but reversible deterioration in their cardiac condition. We conclude that thyrotoxicosis increases right to left shunt in congenital cyanotic heart disease. This effect is profound but reversible. We suggest that when an unexplained rapid deterioration occurs in patients with congenital cyanotic heart disease, thyrotoxicosis should be excluded.