Cardiac systolic time intervals and thyroid hormone levels during treatment of hypothyroidism

Abstract

This study was undertaken to compare results of modern serum thyroid hormone assays with cardiac systolic time intervals (STI) during thyroxine treatment in hypothyroid patients. The patients were assessed clinically (Billewicz index) and the STI and serum thyrotropin (TSH), total and free thyroxine (T₄) and total and free triiodothyronine (T₃) were determined in 16 hypothyroid women (Group I) treated with 50 μg increments of thyroxine, and in 13 women who had a history of thyroid carcinoma and high-dose thyroxine replacement therapy and had elevated thyroid hormone concentrations (Group II). The STI of 24 matched healthy female controls were used for reference of STI.

The pre-ejection period (PEP) index and the PEP/LVET ratio (left ventricular ejection period) were greater in untreated overtly and mildly hypothyroid patients (p < 0.05) than in the controls. During stable thyroxine therapy [mean daily dosage for Group I 137.5 (7.3) μg and for Group II 220 (61) μg] the PEP correlated with serum free T₄ (FT₄), as measured by a two-step method (Spectria^R) (r = -0.55, p < 0.01, n = 29) and total T₄ (r = -0.51, p < 0.05, n = 29), but not with TSH, T₃, FT₃ or FT₄ measured by an analogue method Amerlex-M^R. The TRH test was not valuable in follow-up because of the strong correlation between basal TSH and stimulated TSH values (r = 0.95).

In conclusion, STI are useful for assessment of the thyroid state in untreated hypothyroid patients. Serum TSH becomes normal in the same time as STI and is the best for follow-up. If serum TSH is low and the patient is on stable thyroxine therapy, we recommend serum FT₄ for monitoring thyroxine replacement. Two-step FT₄ assays had the best correlation with STI, which has significance in patients with non-thyroidal illness.

Key Words:

• hypothyroidism
• systolic time intervals
• thyroid hormone assay
• thyrotropin