Effect of surgical stress on serum thyroid hormones in hyperthyroidism

The effect of elective non-thyroidal surgery on serum total and free triiodothyronine (T3), reverse T3 and thyroxine (T4) has been studied in one patient with mild, untreated typical hyperthyroidism, one with mild T3 toxicosis and in four euthyroid patients. The high T3 level of typical hyperthyroidism became normal post-operatively, decreasing to 47% of the mean pre-operative level 24 hours after surgery, while total reverse T3 doubled. In euthyroid subjects T3 decreased to 57 +/- 6 (SEM)% of the pre-operative level 24 hours after surgery, associated with a doubling of reverse T3. The patient with T3 toxicosis, whose surgical stress was less severe, showed a 17% decrease in T3 without change in reverse T3. Because the inverse post-operative changes in T3 and reverse T3 described in euthyroid subjects also occur in typical hyperthyroidism, measurement of serum T3 may be diagnostically unreliable after surgical stress.