Importance of postmortem changes in measurements of thyroid function in studies of sudden infant death syndrome.

Various measurements of thyroid function were made from serum sampled at necropsy by cardiac puncture in adults and compared with the equivalent values before death. There were significant increases in serum total T3 (0.66 before death, 1.14 nmol/l after death), free T3 (0.82 to 1.52 pmol/l), and reverse T3 (1.12 to 1.75 nmol/l); and significant decreases in total T4 (73.7 to 56.6 nmol/l), free thyroxine index (FTI) (83.2 to 56.6), and free T4 (14.2 to 11.9 pmol/l). The increase in serum T3, presumably from T4 deiodination after death, observed in this study may explain the increased T3 concentration reported in serum sampled from cases of sudden infant death syndrome (SIDS).