Abstract
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.
Highlights
Thyroid cancer is a malignant neoplasm originating from the thyroid epithelial tissue and is the most common neoplasm of the endocrine glands [1]
The 2015 guidelines of the American Thyroid Association for the treatment of nodules and differentiated thyroid cancer in adults, which are valid in Poland, were published in January 2016 [6]
All groups were homogeneous in terms of age, hemoglobin, total calcium, potassium, free triiodothyronine levels, and the presence of hypertension
Summary
Thyroid cancer is a malignant neoplasm originating from the thyroid epithelial tissue and is the most common neoplasm of the endocrine glands [1]. Papillary cancer is the most common malignant neoplasm of the thyroid gland, accounting for 60–95% of all malignant thyroid neoplasms, with a peak incidence between the ages of 20 and 40 years [2]. Treatment is a multi-stage process and is highly effective It consists of thyroidectomy, 131I radioiodine therapy, and chronic thyroxine therapy, usually with the aim of full or partial thyroid-stimulating hormone (TSH) suppression in order to minimize the risk of recurrence [7]. Previous studies have shown that endogenous subclinical hyperthyroidism can cause an increased number of supraventricular premature beats; the difference compared to the healthy population was not statistically significant [10]. The incidence of atrial fibrillation increases with age, and a low TSH concentration is an independent risk factor for this arrhythmia [12]
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