BACKGROUND: The thyroid gland, as one of the important endogenous glands of the body, controls the physiological function of other organs by secretion of T3 and T4. Alteration in thyroid hormone concentrations may influence the function of different organs and express various clinical symptoms. OBJECTIVES: The purpose of the present study was to investigate the effects of different environmental and physiological parameters on serum thyroid hormones (T4, T3, FT4 and FT3) concentrations of Arab horses in Khuzestan province. METHODS: Blood samples were collected from 240 horses during four different seasons. The ELISA method was used for hormone assay. RESULTS: The results were analyzed using SAS software. The results indicate that the serum concentrations (Least square means±standard error of mean) of T4, T3, FT4 and FT3 were 45.1±1.4 (nmol/L), 1.5±0.03(nmol/L), 13.6±0.31(pmol/L) and 5.5±0.12 (pmol/L), respectively. The serum levels of the hormones were higher in winter (cold season) than summer (warm season). There was no significant difference between female and male. Foals had higher serum levels of the hormones than other age groups (p<0.05). CONCLUSIONS: The environmental and physiological conditions significantly influenced the serum concentration of hormones as in pregnant horses it was less than non-pregnant females.