AIM: To assess whether levothyroxine treatment improves functional capacity in 
patients with chronic heart failure (New York Heart Association class i-iii) and 
subclinical hypothyroidism.
METHODS: One hundred and sixty-three outpatients with stable chronic heart 
failure followed up for at least 6 months were enrolled. A physical examination 
was performed, and laboratory tests including thyroid hormone levels, Doppler 
echocardiogram, radionuclide ventriculography, and Holter monitoring were 
requested. Functional capacity was assessed by of the 6-min walk test. Patients 
with subclinical hypothyroidism were detected and, after undergoing the s6-min 
walk test, were given replacement therapy. When they reached normal thyrotropin 
(TSH) levels, the 6-min walk test was performed again. The distance walked in 
both tests was recorded, and the difference in meters covered by each patient 
was analyzed.
RESULTS: Prevalence of subclinical hypothyroidism in patients with heart failure 
was 13%. These patients walked 292±63m while they were hypothyroid and 350±76m 
when TSH levels returned to normal, a difference of 58±11m (P<.011). Patients 
with normal baseline TSH levels showed no significant difference between the 2 
6-min walk tests.
CONCLUSIONS: Patients with chronic heart failure and subclinical hypothyroidism 
significantly improved their physical performance when normal TSH levels were 
reached.