Although muscle metabolism and exercise capacity seem to be affected in patients 
with subclinical hypothyroidism, there is little evidence indicating improvement 
of the exercise tolerance due to levothyroxine (L-T(4)) replacement. The aim of 
the present study was to verify possible cardiopulmonary changes during exercise 
in patients with subclinical hypothyroidism on L-T(4) replacement with a normal 
serum TSH for six months. Twenty-three patients with subclinical hypothyroidism 
were randomized into treated (no.=11) and untreated (no.=12) patients. A 
cardiopulmonary test was performed with a treadmill, using the modified Balke 
protocol. Heart rate, oxygen uptake, minute ventilation and other 
cardiopulmonary parameters were assessed at the 5th minute of exercise. FT4 
levels increased while TSH normalized after hormone replacement. Oxygen uptake 
decreased significantly after hormone replacement (24.1+/-6.3 vs 17.1+/-4.2 ml x 
kg x min(-1); p=0.03).Minute ventilation also showed an enhanced performance in 
treated patients (28.0+/-8.1 vs 23.5+/-5.6 l x min(-1); p=0.03), as did the 
heart rate (128+/-17 vs 121+/-17 bpm; p=0.03). There were no changes in the 
untreated group. The results demonstrate that submaximal cardiopulmonary 
exercise performance improved after six months of TSH normalization and this 
improvement can help enhance the ability to carry out daily life activities in 
patients with subclinical hypothyroidism.