The treatment of hypothyroidism in patients undergoing coronary artery bypass 
surgery is a difficult clinical problem. To determine perioperative thyroid 
replacement therapy in patients with hypothyroidism, plasma total thyroxine 
(T4), total triiodothyroxine (T3), free T4, free T3 and thyroid-stimulating 
hormone levels were measured preoperatively and at 1, 2, 3, 7, and 14 days after 
operation in 9 patients with hypothyroidism and were compared with levels in 14 
patients with normal thyroid function who underwent coronary bypass surgery. In 
the normal control group, total T4 decreased to its lowest level on the 1st 
postoperative day and then increased gradually to the preoperative level at 7 
days. Total T4 remained within the normal range throughout the entire 
postoperative course. In 6 patients with hypothyroidism who were treated with 
thyroid hormone before surgery, total T4 decreased immediately after operation 
and only increased after starting thyroid replacement therapy. In 3 hypothyroid 
patients without prior thyroid replacement, total T4 showed a change similar to 
patients in the control group but remained below the normal range until starting 
thyroid replacement therapy. Coronary bypass surgery was performed safely in 
patients with hypothyroidism. Preoperative thyroid replacement with suboptimal 
doses was safe in patients with severe hypothyroidism. Adequate postoperative 
thyroid replacement was achieved in all patients without complications.