Minimal cardiac effects in asymptomatic athyreotic patients chronically treated with thyrotropin-suppressive doses of L-thyroxine

Biondi, Fazio, and colleagues recently reported that long term T₄ treatment to suppress serum TSH markedly affects cardiac function. T₄- treated patients had more symptoms [12.2 ± 3.9 (±SD) vs. 4.2 ± 2.3 by quantitative questionnaire], higher mean heart rate, increased incidence of atrial extrasystoles, increased interventricular septal thickness and left ventricular mass index [LVMi), and significant diastolic dysfunction. The severity of cardiac abnormalities was highly correlated with scores of a rating scale used for assessing symptoms of thyrotoxicosis. We have duplicated their studies in 17 athyreotic patients (mean age, 45 ± 10 yr; range, 27-63 yr) without heart disease or hypertension whose dose of T₄ was titrated to suppress serum TSH to less than 0.01 μU/mL. The mean duration of T₄ treatment was 9.2 ± 5.4 yr. Controls were healthy volunteers matched for sex and age (±3 yr). The mean T₄ dose was 2.8 ± 0.9 μg/kg (0.192 ± 0.058 mg/day). By questionnaire, patients had minimalsymptoms, although their symptom score was significantly greater than the control value (4 ± 3 vs. 2 ± 1; P < 0.05; maximum score, 36). No differences in mean heart rate or in atrial or ventricular extra-systoles were noted. In patients, indexes of systolic and diastolic function and interventricular septal thickness were similar to control values. The mean LVMi was normal in both groups. However, the mean LVMi in patients (117 ± 35 g/m²) was higher than that in controls (92 ± 31; P < 0.05). In conclusion, patients were minimally affected by TSH- suppressive doses of T₄. They had few symptoms and no increase in extrasystoles or basal heart rate. Based on current knowledge, the increase in LVMi observed in patients without associated significant systolic or diastolic abnormalities does not have clinical or prognostic importance. Therefore, in the absence of symptoms of thyrotoxicosis, patients treated with TSH-suppressive doses of L-T₄ may be followed clinically without specific cardiac laboratory studies.