EFFECT OF FOOD-DEPRIVATION ON BASE-LINE IODOTHYRONINE AND CORTISOL CONCENTRATIONS IN HEALTHY, ADULT HORSES

Six healthy, adult horses, with normal (mean +/- SEM) baseline serum c oncentrations of total triiodothyronine (T3, 1.02 +/- 0.16 nmol/L), fr ee T3 (FT3, 2.05 +/- 0.33 pmol/L), total thyroxine (T4, 19.87 +/- 1.74 nmol/L), free T4 (FT4, 11.55 +/- 0.70 pmol/L), total reverse T3 (rT3, 0.68 +/- 0.06 nmol/L), and cortisol (152.75 +/- 17.50 nmol/L), were j udged to be euthyroid on the basis of response to a standardized thyro id-stimulating hormone response test. Serum concentrations of T3, FT3, T4, FT4, rT3, and cortisol were determined immediately before and eve ry 24 hours during a 4-day period of food deprivation, when water was available ad libitum. Similar variables were measured 72 hours after r efeeding. Decreases (to percentage of baseline, prefood deprivation va lue) in circulating T3 (42%), T4 (38%), FT3 (30%), and FT4 (24%) conce ntrations were maximal after 2, 4, 2, and 4 days of food deprivation, respectively (P < 0.05). Increases (compared with baseline, prefood de privation value) in rT3 (31%) and cortisol (41%) concentrations were m aximal after 1 and 2 days of food deprivation, respectively (P < 0.05) . Refeeding resulted in increase in serum T4 and FT4, and decrease in rT3 and cortisol concentrations toward baseline values, after 72 hours (P < 0.05). Refeeding did not effect a return of T3 or FT3 concentrat ion to baseline values after 72 hours (P < 0.05). Food deprivation app ears to cause changes in serum concentrations of T3, FT3, T4, FT4, rT3 , and cortisol in horses that are similar to those in human beings. Th is effect of food deprivation should be considered when results of ser um thyroid hormone and cortisol assays are interpreted in the face of clinical disease. These results further emphasize the invalidity of ma king a clinical diagnosis of hypothyroidism on the basis of baseline, serum thyroid hormone concentrations in horses, especially if the hors es have been anorectic or inappetent.