The beneficial effect of L-thyroxine on cardiovascular risk factors, endothelial function, and quality of life in subclinical hypothyroidism: randomized, crossover trial.

CONTEXT: Subclinical hypothyroidism (SCH) is defined as raised serum TSH levels with circulating thyroid hormones within the reference range. It is uncertain whether treatment of SCH with L-thyroxine improves cardiovascular (CV) risk factors and quality of life.

OBJECTIVE: The objective of the study was to assess CV risk factors and patient-reported outcomes after treatment.

DESIGN: This was a randomized, double-blind, crossover study of L-thyroxine and placebo.

SETTING: The study was conducted with community-dwelling patients.

PATIENTS: One hundred patients [mean age (sd) 53.8 (12) yr, 81 females] with SCH [mean TSH 6.6 (1.3) mIU/liter] without previously treated thyroid or vascular disease.

INTERVENTION: Intervention consisted of 100 microg L-thyroxine or placebo daily for 12 wk each.

MEASUREMENTS: Primary parameters were total cholesterol (TC) and endothelial function [brachial artery flow-mediated dilatation (FMD)], an early marker of atherosclerosis. Patient-reported outcomes were also assessed.

RESULTS: L-thyroxine treatment reduced TC (vs. placebo) from 231.6 to 220 mg/dl, P < 0.001; low-density lipoprotein cholesterol from 142.9 to 131.3 mg/dl, P < 0.05; waist to hip ratio from 0.83 to 0.81, P < 0.006; and improved FMD from 4.2 to 5.9%, P < 0.001. Multivariate analysis showed that increased serum free T(4) level was the most significant variable predicting reduction in TC or improvement in FMD. Furthermore, the symptom of tiredness improved on L-thyroxine therapy, but other patient-reported outcomes were not significantly different after correction for multiple comparisons.

CONCLUSION: SCH treated by L-thyroxine leads to a significant improvement in CV risk factors and symptoms of tiredness. The CV risk factor reduction is related to the increased level of achieved free T(4) concentration.