Design-Cross sectional study of patients prescribed thyroxine long term (n = 11), patients with thyrotoxicosis studied at presentation (n = 23), compared with controls (n = 25); longitudinal study of patients with thyrotoxicosis studied at presentation and serially after beginning antithyroid drug treatment (n = 23). Methods-24 h ambulatory monitoring of pulse and blood pressure, echocardiography, forearm plethysmography, and autonomic function tests. Results-Long-term thyroxine treatment in doses that reduced serum thyrotrophin to below normal had no effect on blood pressure, heart rate, left ventricular systolic function or stroke volume index, but was associated with an 18-4% increase in left ventricular mass index (mean (SEM) 101'9 (3.09) g/m' v controls 86-1 (4.61), P < 0.01). Thyroxine treatment, like thyrotoxicosis, had no effect on tests of autonomic function. Untreated thyrotoxicosis resulted in pronounced changes in systolic and diastolic blood pressure and an increase in heart rate during waking and sleep. Patients with thyrotoxicosis at presentation had an increase in left ventricular systolic function (ejection fraction 70 5 (1P66)% v 65 4 (1.79), P < 0 01; fractional shortening 40 4 (1.54)% v 35 6 (1.46), P < 0.01), increased stroke volume index (45.9 (2.4) mu/m2 v 36-6 (1.7), P < 0.001), and an increase in forearm blood flow, and decrease in vascular resistance. They had a similar degree of left ventricular hypertrophy to that associated with thyroxine treatment (99.3 (4.03)

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