Context: Overt hypo- and hyperthyroidism are associated with cardiac disease, whereas this relation is more uncertain regarding subclinical thyroid dysfunction.

Objective: The objective was to assess the relation between serum TSH level and cardiac function.

Design: The design was a cross-sectional epidemiological study and a nested case-control study.

Setting: The study was performed at a university hospital.

Subjects: 2035 subjects were included in the epidemiological study and 204 subjects in the nested case-control study (serum TSH < 0.50, 0.50 - 3.49, and 3.50 - 10.0 mIU/L in 20, 118, and 66 subjects, respectively, all with normal serum free T₄ and free T₃ levels).

Main outcome measures: Left ventricular mass by body surface area (LVMI) and indices of left ventricular function, as assessed by conventional and pulsed wave tissue Doppler (PWTD) echocardiography, were recorded.

Results: No significant relation was found between serum TSH level and LVMI. In the nested case-control study, the subjects with serum TSH 3.50 - 10.0 mIU/L had no signs of cardiac dysfunction. However, the PWTD data showed higher velocities at all measurement sites in the subjects with serum TSH < 0.50 mIU/L as compared to the euthyroid group.

Conclusions: With the possible exception of overt hypo- and hyperthyroidism, there is no significant association between serum TSH level and LVMI. Subjects with subclinical hypothyroidism, where the mean serum TSH level is slightly above the reference range, appear to have normal cardiac function, whereas subjects with serum TSH levels < 0.5 mIU/L appear to have changes in myocardial velocities detected by PWTD.