Thyroid hormone: relevance to xenotransplantation

Background It has been well documented that the level of serum/plasma free triiodothyronine ( fT 3) falls rapidly following brain death or during certain surgical procedures, for example, heart surgery carried out on cardiopulmonary bypass. The level in patients following cardiopulmonary bypass usually recovers within 2 days. Methods We have measured serum fT 3 in healthy naïve baboons (n = 31), healthy naïve monkeys (n = 5), and after pig‐to‐baboon heterotopic heart xenotransplantation (xenoTx) (Group 1, n = 9), orthotopic liver xenoTx (Group 2, n = 10), artery patch xenoTx (Group 3, n = 9), and in monkey‐to‐monkey heterotopic heart alloTx (Group 4, n = 5). Results The mean level of fT 3 in healthy naïve baboons was 3.1 ± 0.9 pg/ml and in healthy naïve monkeys was 2.6 ± 0.3 pg/ml. Following pig heart, liver, and artery patch xenoTx and monkey heart alloTx, there was an immediate rapid fall in fT 3 level. Recovery of fT 3 was more rapid in Groups 3 and 4 than in Groups 1 and 2. In Group 1, within 4 days fT 3 had recovered, but only to the lower limit of normal range, where it remained throughout follow‐up (for up to 42 days). In Group 2, no recovery was seen during the 7 days of follow‐up. In immunosuppressed baboons with pig patch grafts that received IL ‐6R blockade (n = 2), the fT 3 tended to rise higher than in those that received no IL ‐6R blockade (n = 6). Conclusions Following operative procedures, there is a dramatic fall in serum fT 3 levels. The persistent low level of fT 3 after pig heart and liver xenoTx may be associated with a continuing inflammatory state. We suggest that consideration should be given to the replacement of T3 therapy to maintain normal fT 3 levels, particularly in nonhuman primates undergoing orthotopic pig heart or liver xenoTx.