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Cardiac Function, Physical Exercise Capacity, and Quality of Life during Long-Term Thyrotropin-Suppressive Therapy with Levothyroxine: Effect of Individual Dose Tailoring

Institute of Cardiology (G.M., M.G.P., A.B., M.L.) and Endocrinology, Department of Medical Sciences (R.C., L.P., F.P., S.M.), University of Cagliari, 09124 Cagliari, Italy

Address all correspondence and requests for reprints to: Prof. Giuseppe Mercuro, M.D., Institute of Cardiology, University of Cagliari, Via S. Giorgio 12, 09124 Cagliari, Italy. E-mail: mercuro{at}pacs.unica.it

As recently claimed, TSH-suppressive therapy with L-T₄ may have adverse effects on the heart, but these results have not been consistently confirmed. We assessed cardiac function by clinical, echocardiographic, and ergometabolic criteria in 19 patients (16 women and 3 men) receiving long term L-T₄ at a fixed daily dose ranging from 1.8–4.0 µg/kg. The results showed significant alterations in several cardiac parameters suggestive of subclinical hyperthyroidism. In particular, intraventricular septum thickness (10.0 ± 1.4 vs. 8.1 ± 1.1 mm), left ventricular posterior wall thickness (9.4 ± 1.5 vs. 8.1 ± 1.1 mm), end-diastolic dimension (47 ± 4 vs. 44 ± 3 mm), and left ventricular mass index (102 ± 15 vs. 75 ± 15 g/m²) were significantly increased compared to values in age- and sex-matched euthyroid controls. Exercise tolerance (expressed as maximal tolerated workload; 102 ± 14 vs. 117 ± 12 watts), maximal O₂ achieved at peak exercise (maximum O₂, 17.3 ± 3.3 vs. 21.9 ± 2.5 mL/min·kg), and anaerobic threshold (expressed as a percentage of O₂max, 46.5 ± 8.4 vs. 56.2 ± 6.6) were significantly reduced in L-T₄-treated patients. The L-T₄ dose was then reduced to the minimal amount able to keep the serum TSH concentration at 0.1 mU/L or less in 7 patients who were reevaluated 6 months after the initial study. This individual tailoring of the TSH-suppressive L-T₄ dose was in all cases associated with normalization of all echocardiographic and ergometabolic parameters. In conclusion, our findings show that abnormalities of heart morphology associated with impaired exercise performance occur as a consequence of long term therapy with fixed TSH-suppressive doses of L-T₄, but that these abnormalities improve or disappear after careful tailoring of TSH-suppressive therapy.

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