Haemodynamic changes following treatment of subclinical and overt hyperthyroidism

Hyperthyroidism has profound effects on the cardiovascular system, including reduced systemic vascular resistance (SVR) due to relaxation of vascular smooth muscle cells, enhanced heart rate (HR) and cardiac output (CO) due to an increase in cardiac diastolic relaxation, contractility and heart rate. Subclinical hyperthyroidism is characterised by reduced serum TSH levels despite free thyroxine (T4) and tri-iodothyronine (T3) estimates within the reference range, in subjects with no obvious symptoms of hyperthyroidism. We measured haemodynamic changes (using impedance cardiography) in subjects with endogenous subclinical hyperthyroidism in order to elucidate whether these patients had signs of excess thyroid hormone at the tissue level.

The patients were otherwise healthy women with a multinodular goitre (n=6; age 47-81 years; serum TSH 0.006-0.090 mU/l and normal free T4 and T3 estimates), studied before and after normalisation of TSH (0.280-1.120 mU/l) by means of radioiodine treatment, and they were compared with 9 overt hyperthyroid patients (2 with multinodular goitre and 7 with Graves' disease) in the untreated state and after euthyroidism had been obtained.

Treatment of the subclinical hyperthyroid women resulted in 11% reduction in HR (P<0.02), 19% reduction in CO from (means+/-s.d.) 6.93+/-2.15 l/min to 5.58+/-1.94 l/min (P<0.05), and 30% increase in SVR (P<0.02). Similar but more pronounced changes were seen in the hyperthyroid group: 17% reduction in HR, 25% reduction in CO and 46% increase in SVR (all at least P<0.05). Taking all 15 patients together, thyroid function (as measured by free T3 index (FT3I) or TSH) correlated significantly to the haemodynamic parameters as follows: the higher the thyroid function the lower the mean arterial pressure and SVR, and the higher the CO and central aortic compliance (stroke volume/pulse pressure) (P<0.05). Plasma norepinephrine increased significantly after treatment of the overt hyperthyroid patients, whereas epinephrine did not change, and no changes were seen among subclinical hyperthyroid patients.

Treatment of endogenous subclinical hyperthyroidism resulted in significant changes in several haemodynamic parameters regarding the heart and the vascular system, compatible with some degree of excess tissue exposure to thyroid hormones in the untreated state. Our data favour more aggressive treatment of these patients, and endogenous subclinical hyperthyroidism might be regarded as a mild form of hyperthyroidism.