Thyroid hormone alterations (known as the "sick-euthyroid syndrome") are common 
following major surgery, but the time course for appearance and recovery from 
these alterations has not previously been longitudinally studied in a large 
group of surgical patients. The authors prospectively studied 59 patients 
undergoing major surgery (coronary artery bypass grafting, pneumonectomy, or 
subtotal colectomy). Compared with preoperative values, the mean serum T4, T3, 
free T3, and TSH concentrations decreased significantly (p less than 0.05) 
following surgery. Serum reverse T3 and T3 resin uptake index increased, while 
free T4 levels remained unchanged. These changes were seen within 6 hours of 
surgery and normalized by 1 week after surgery. Although the serum TSH response 
to TRH was normal before and after surgery in 56 of the 59 patients, the maximal 
TRH-induced increase in serum TSH and the integrated serum TSH response to TRH 
were suppressed in the early perioperative period. This postoperative TSH 
suppression correlated with elevated postoperative plasma dopamine 
concentrations (r = 0.57, p less than 0.05). Three patients with compensated 
primary hypothyroidism were detected in the study and represent the first 
documentation of serial thyroid hormone and TSH levels in hypothyroid patients 
undergoing major surgery. These patients had similar changes in thyroid hormone 
values compared with euthyroid patients. The serum TSH response to TRH was 
suppressed into the normal range in two of these patients on the day following 
surgery. The authors conclude that the sick-euthyroid syndrome occurs within a 
few hours of major surgery and remits with convalescence. Postoperative 
decreases in serum TSH may mask the diagnosis of hypothyroidism. Surgical 
consultants should be aware of these rapid postoperative changes so that thyroid 
function tests are properly interpreted in patients who have undergone major 
surgery.