Endocrine Abstracts

The suppression of serum TSH is a marker for increased risk of vascular mortality or death due to other causes are unknown. Nevertheless, patients who was operated on account of differentiated carcinoma of thyroid, had to take high doses of thyroxine for a long time to achieve suppression of TSH<0.1 mU/l. Heart rate variability (HRV), a method of mathematical analysis of heart rhythm (computer analysis of 24 h Holter monitoring), was used to assess function of the autonomic nervous system and its central control. We recruited 18 patients (5 men, 13 women, age 48.22±2.02 years), who received suppressive dose of thyroxine and had a level TSH≤ 0.01 mU/l. We found a considerable difference of heart rate variability which turned us to divide patients into two groups. 1-group (n=14) with significant lowering of time domain measurements and frequency domain measurements comparing to health control. 2-group (n=4) characterized with time domain measurement close to health control but significantly (P<0.01) increased frequency domain measurements comparing both to 1-group and health control. Also longevity of thyroxine suppressive therapy intake varied − 1 group − 3–7 years, 2-group – 0.5–1 year. We consider that obtained results are stages of the same process. During short-term use of high doses of thyroxine (as adaptation period) take place significant activation of sympathetic and parasympathetic parts of autonomic nervous system and neurohumoral system. Afterwards, a period of dysadaptation begins with considerable lowering of heart rate variability. Also we found significant prolongation of corrected Q-T interval (QTc) in patients with long time suppression with thyroxine. QTc, together with low values of SDNN and HRV-triangular index determine increased risk of severe complications and arrhythmic death. Follow-up of this cohort this all allow us to identify high-risk groups who should be targeted for therapeutic intervention.