Serum cortisol and thyroid hormone concentrations and survival in foals born from mares with experimentally induced ascending placentitis

BACKGROUND: There are few publications on occurrence of nonthyroidal illness syndrome in foals and on the prognostic value of cortisol and thyroid hormone (TH) concentrations in newborn foals. OBJECTIVES: To determine serum cortisol and TH concentrations (total and free thyroxine: T(4) and (F)T(4); total and free triiodothyronine: T(3) and (F)T(3)) in foals born from mares with placentitis, to determine their association with survival, and their use as prognostic markers. ANIMALS: A cohort of 29 newborn foals comprising 5 Control, 14 Low‐risk, and 10 Sick foals were evaluated over the first week of life. METHODS: In this prospective study foals born to mares with experimentally‐induced placentitis were assigned to Low‐risk or Sick groups while foals born to control mares were classified as Control based on clinical findings. Foals were also classified as Term (n = 13), Dysmature (n = 7), or Premature (n = 9), and survival rate was recorded. Serum cortisol and TH hormone concentrations were measured at 0, 12, 24, 48, and 168 hours of life. RESULTS: Sick non‐surviving foals had lower (P < .05) T(3) : cortisol ratio at 12 (3.68 ± 1.06 versus 18.58 ± 2.78), 24 (5.47 ± 2.34 versus 23.40 ± 3.82), and 48 (10.47 ± 6.29 versus 26.6 ± 2.90) hours of life when compared to Sick surviving foals and lower (P < .05) T(4) : cortisol ratio at 12 (75.12 ± 21.71 versus 414.47 ± 58.47) and 24 hours (127.83 ± 55.21 versus 430.87 ± 80.31) after birth than Sick surviving foals. CONCLUSION AND CLINICAL IMPORTANCE: Placental infections can impair fetal thyroid function. Low T(3) : cortisol and T(4) : cortisol ratios seem to be good prognostic markers in newborn foals.