Results of thyroid function tests in dogs with systemic nonthyroidal illness commonly mimic those found in hypothyroidism. Alterations to thyroid function were evaluated in this study of six healthy male dogs experimentally given Escherichia coli O111:B4 endotoxin (1 µg/kg) every 12 hours for 3.5 days and followed during illness and recovery over a 24-day period. Serum concentrations of thyroxine (T4), 3,5,3' triiodothyronine (T3), 3,3',5' triiodothyronine (rT3), free T4 (fT4), endogenous canine thyroid-stimulating hormone (TSH), and tumor-necrosis factor-a (TNF-a) activity were measured before (day -1, baseline), during (days 0 to 3), and after (days 4 to 24) IV administration of endotoxin. A decrease in serum T4 and T3 concentrations and an increase in rT3 concentration indicated impaired secretion and metabolism of thyroid hormones during the course of the infection; however,all returned to reference ranges by day 24. No significant change in TSH was detected, and TNF-a activity significantly increased 4 hours after administration of endotoxin but not subsequently.

COMMENTARY: A persistent decrease in serum T4 concentration in this study of dogs withnonthyroidal disease indicates that caution should be used in interpreting serum T4 levels after resolution of an illness in dogs.

Endotoxin-induced nonthyroidal illness in dogs. Panciera DL, Ritchey JW,Ward DL. AM J VET RES 64:229-234, 2003.