Context: Subclinical hypothyroidism (SCH) is associated with increased risk of cardiac disease; its impact on arterial function is less clear.

Objective: Assessment of arterial and cardiac function.

Design: A 6 month controlled observational study using pulse wave analysis and tissue Doppler dobutamine stress echocardiography.

Setting: Thyroid clinic.

Patients: 19 female SCH patients recruited from the thyroid clinic [age 49.2 ± 3.8 yr; body mass index (BMI) 29.9 ± 6.7 kg/m (2)) and 10 female controls (age 50.2 ± 3.4 yr; BMI 29.7 ± 7.2 kg/m²) with raised TSH, normal Free T4 and no cardiovascular disease.

Interventions: Incremental doses of L-thyroxine.

Main outcome measures: Indices of vascular stiffness and left ventricular echocardiographic function. Before data collection; any improvement with L-thyroxine?

Results: Baseline augmentation gradient was elevated in SCH compared with controls (10.3 ± 5.1 [SD] mmHg vs. 8.0 ± 4.2, P < 0.05); when euthyroid (mean thyroxine dose 114 mcg/day) it fell to 8.8 ± 5.3 mmHg (P < 0.05). Heart rate corrected augmentation index was 26.7 ± 9.9 vs. 18.8 ± 9.9%, (P < 0.02); falling to 19.7 ± 9.6% (P < 0.001) after treatment. Time of travel of the reflected wave was 139.3 ± 11.7ms cf. 141.5 ± 8.8ms in controls (P < 0.05) increasing to 144.9 ± 11.9 ms (P < 0.05). There were no differences in resting global, regional left ventricular function or regional myocardial velocities during maximal dobutamine stress between SCH patients and controls, or in treated patients compared with baseline.

Conclusions: Arterial stiffness was increased in SCH and improved with L-thyroxine, which may be beneficial, whereas myocardial functional reserve was similar to controls and remained unaltered after treatment.