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.