[Changes of the Heart Rhythm in Imbalance of Thyroid Gland Hormones and its Morphology in Outbred Rats].

Aim: To analyze the relationship between the occurrence of atrial fibrillation (AF) and thyroid dysfunction caused by low concentrations of free triiodothyronine (FT3), free triiodothyronine and free thyroxine (FT3 and FT4), and high concentrations of free thyroxine (FT4) with normal values of thyroid-stimulating hormone (TSH) in experiments on outbred rats.
Material and Methods: The pathogenesis of AF with low concentrations of FT3, FT3 and FT4, and a high concentration of FT4 was studied in an experiment on 146 outbred rats. In the experiment, hypothyroidism, euthyroidism, and thyrotoxicosis were modeled by changing the concentrations of thyroid hormones, and the effect of these conditions on ECG and the incidence of AF was evaluated.
Results: The types of the hypothalamic-pituitary-thyroid system response to the L-thyroxine administration in experimental hypothyroidism were identified. A relationship was determined between AF paroxysms and threshold values of thyroid hormones at low concentrations of FT3, FT3 and FT4, and a high concentration of FT4 with normal TSH values. It was shown that achieving the euthyroid status did not reduce the incidence of AF paroxysms by more than 70.0%.
Conclusion: In AF with low FT3, low FT3 and FT4, and high FT4, the dose of L-thyroxine must be adjusted.