Cardiopulmonary Bypass: ‘A Low T ₄ and T ₃ Syndrome’ with Blunted Thyrotropin (TSH) Response to Thyrotropin-Releasing Hormone (TRH)

Abstract

Thyroid hormone serum concentrations, the thyrotropin (TSH) and prolactin (PRL) response to thyrotropin-releasing hormone (TRH) were evaluated in patients undergoing cardiopulmonary bypass (CPB) conducted in hypothermia. During CPB a marked decrease of thyroxine (T<sub>4</sub>) and triiodothyronine (T<sub>3</sub>) concentration with a concomitant increase of reverse T<sub>3</sub> (rT<sub>3</sub>) were observed similarly to other clinical states associated with the ‘low T<sub>3</sub> syndrome’. Furthermore, in the present study elevated FT<sub>4</sub> and FT<sub>3</sub> concentrations were observed. In a group of patients, TRH administered during CPB at 26°C elicited a markedly blunted TSH response. In these patients, PRL concentration was elevated but did not significantly increase after TRH. The increased concentrations of FT<sub>4</sub> and FT<sub>3</sub> were probably due to the large doses of heparin administered to these patients. Thus, the blunted response of TSH to TRH might be the consequence of the elevation of FT<sub>4</sub> and FT<sub>3</sub> in serum, however, other factors might play a role since also the PRL response to TRH was blocked.