L-Thyroxine Beneficial in Subclinical Hypothyroidism

NEW YORK (Reuters Health) Jun 08 - Treatment of adults with subclinical hypothyroidism with L-thyroxine has beneficial effects on cardiovascular risk factors, endothelial function and quality of life, clinicians from the UK report.

In a randomized crossover study, they treated 100 individuals with subclinical hypothyroidism without previously treated thyroid or vascular disease with 100 g L-thyroxine or placebo daily for 12 weeks each. The subjects, whose average age was 53.8 years, had mean TSH levels of 6.6 (1.3 mIU/liter).

In the June Journal of Clinical Endocrinology and Metabolism, Dr. Salman Razvi from Queen Elizabeth Hospital, Gateshead and colleagues report that L-thyroxine therapy was associated with "significant, although modest, improvement across a wide spectrum of cardiovascular risk factors."

Compared to placebo, L-thyroxine treatment reduced total cholesterol from 231.6 to 220 mg/dL (p < 0.001 and LDL cholesterol from 142.9 to 131.3 mg/dL (p < 0.05). If sustained long term, this reduction in LDL-C, the researchers estimate, "would result in a relative reduction in 10-year cardiovascular mortality of about 10%."

"Rather surprisingly, when considering the short treatment period," L-thyroxine was also associated with a reduction in waist to hip ratio from 0.83 to 0.81 (p < 0.006). Active treatment also improved brachial artery flow-mediated dilatation (FMD), a validated surrogate marker for coronary artery endothelial function, from 4.2 to 5.9% (p < 0.001).

In a multivariate analysis, an increase in serum free T4 level was the most significant predictor of a reduction in total cholesterol or improvement in FMD, the authors report.

Fewer subjects reported tiredness while on L-thyroxine and there was a trend toward improvement in the perceived negative impact of hypothyroidism on sex life, "which is interesting and warrants further research into the underlying mechanism," Dr. Razvi and colleagues write.

"However, this is not an isolated finding, as L-thyroxine has been shown to improve impaired sexual function and performance in men with overt hypothyroidism," they note. "This may be due to the effect of thyroid hormones on psychological aspects (e.g. reducing tiredness)."

Dr. Razvi and colleagues write that long term studies are needed to determine whether the apparent short term benefits of L-thyroxine therapy in subclinical hypothyroidism will translate into reduction in cardiovascular illness and death.