L-Thyroxine Suppressive Therapy and Autonomic Nervous System Control in Patients with Thyroid Disease

Abstract

In our experience, 23 patients (20 female, 3 male, aged 49.6 8 1.5 years; BMI 26.6 8 0.9, group A) undergoing suppressive therapy (L-T4 doses 0.8–2.3 g/kg/day) for multinodular goiter (thyroid-stimulating hormone ! 0.3 IU/ml during the last year) were compared to 14 patients with nodular goiter not assuming L-T4 (13 female, 1 male, aged 50 8 2.9 years; BMI 26.9 8 1.4, group B). All patients were normotensive and no other drugs were given, except for L-T4. Basal ECG was recorded and autonomic nervous system activity assessment tests [lying to standing (LS), standing to lying (SL), cough test (CT), deep breathing (DB) and postural hypotension] were performed by Cardionomic (Medimatica, Italy) [5] . In group A, serum free T4 levels were higher (19.2 8 1.2 vs. 13.6 8 1.2 pmol/l, p ! 0.001) and thyroid-stimulating hormonelevels were lower than in group B (0.15 8 0.03 vs. 1.15 8 0.24 IU/ml, p ! 0.001), while free T3 levels did not differ (5.0 8 1.5 vs. 4.5 8 3.1 pmol/l). At ECG, no differences were observed in QT and QTc, while heart rate was slightly higher in group A (73.9 8 2 vs. 67.5 8 2 beats/min, n.s.). Blood pressure did not differ between the two groups both in clinoand orthostatism and postural hypotension was never observed. Among the 23 patients Dear Sir, We read with great interest the article by Murialdo et al. [1] entitled ‘Effects of recombinant human thyrotropin on heart rate variability and blood pressure in patients on L -thyroxine-suppressive therapy for differentiated thyroid carcinoma’. The authors investigated the autonomic control of heart rate variability in their patients and concluded, as recently reported in a similar series [2] , that a decreased sympathetic activity and impaired sympathovagal balance with preserved vagal tone are often present in patients treated with suppressive doses of L -thyroxine (L-T4) for thyroid cancer. We agree with their statement and, on the basis of our experience, we would like to add some comments about this interesting issue. Alterations of cardiac rhythm are not so rare in subclinical hyperthyroidism and changes in its autonomic control have been previously reported [2–4] . In their article, Murialdo et al. [1] analyzed the autonomic control of heart rate variability in subjects receiving chronic L-T4 therapy after total thyroidectomy and 131 I-therapy for differentiated thyroid carcinoma. Spectral analysis of heart rate variability detected an increased sympathetic activity and impaired sympathovagal balance in the presence of preserved vagal tone. Published online: March 9, 2006 HORMONE RESEARCH