Occurrence of Arrhythmias in Women with Thyroid Cancer Receiving Suppressive Doses of Levothyroxine

Aim of the study: To assess the occurrence of cardiac arrhythmias caused by high doses of levothyroxine in patients with thyroid cancer with subclinical hyperthyroidism. Materials and Methods: This prospective study included 98 women divided into three groups according to plasma thyroid stimulating hormone (TSH) concentration: <0.1 µU/mL, 0.1–0.39 µU/mL, or 0.4–4.0 µU/mL (control group). All participants underwent laboratory tests and an electrocardiography (ECG) Holter test to assess their heart rate and the occurrence of arrhythmias. Statistical analysis assessed differences between groups in all clinical parameters and factors influencing the occurrence of arrhythmias. Results: There were no differences between groups in the maximum, average, or minimum heart rate or in the incidence of the studied cardiac arrhythmias. Heart rate in women with a TSH concentration of <0.1 µU/mL depended on age and the presence of arterial hypertension, and heart rate in women with a TSH concentration of 0.4–4.0 µU/mL depended on free triiodothyronine concentration and the presence of arterial hypertension; no relationship was identified for women with a TSH concentration of 0.1–0.39 µU/mL. One-way logistic regression analysis did not identify any factors influencing the occurrence of arrhythmias. Conclusions: While maintaining normal free triiodothyronine levels, the use of suppressive doses of levothyroxine after thyroidectomy for differentiated thyroid cancer does not induce clinically significant arrhythmias or affect average heart rate. None of the studied clinical parameters influenced the risk of arrhythmia.