Endogenous subclinical hyperthyroidism: Metabolic and cardiac parameters

Subclinical hyperthyroidism (SH) is defined by suppressed TSH and normal levels of thyroid hormones. Endogenous subclinical hyperthyroidism (ESH) is probably less common than exogenous SH. Adverse effects of SH due to exogenous administration of thyroxine have been well studied, while the impact of ESH on the cardiovascular system and metabolic parameters remains controversial.

In a cross-sectional study, we examined patients with endogenous clinical hyperthyroidism (ECH; n=20), ESH (TSH<0.1 muU/mL, n=25), and mild ESH (TSH=0.1-0.3 muU/mL, n=32), as well as healthy controls (n=50). Biochemical and metabolic parameters influenced by thyroid hormones were assessed and cardiac parameters were studied using echocardiography and 24-hour ECG-blood pressure monitoring.

Biochemical and metabolic parameters did not differ significantly between ESH and healthy subjects. The ECH group had significantly higher sex hormone-binding globulin, osteocalcin, and carboxy-terminal telopeptide levels than healthy subjects. No significant differences were noted in echocardiographic parameters between ESH patients and healthy subjects. The ECH group had a significantly higher heart rate, cardiac output, and cardiac index than the control group, as well as end-diastolic and end-systolic diameters of the left ventricle, and end-diastolic and end-systolic volumes of the left ventricle. The 24-hour ECG-blood pressure monitoring parameters did not differ significantly either between SH and healthy subjects while, in the ECH group, mean heart rate, maximum heart rate, and mean tachycardia episodes were significantly increased.

Only subjects with ECH showed differences in metabolic and cardiac parameters from controls, while no significant effects were noted in the endogenous subclinical forms.