Fecal prevalence of Escherichia coli O157 in ruminants is highest in the summer months and decreases to low or undetectable levels in the winter. We hypothesize that the seasonal variation of this pathogen is a result of physiological responses within the host animal to changing day length. The thyroid is an endocrine gland known to respond to changing day length. Two experiments were conducted to determine if a hyperthyroid status would initiate fecal shedding of E. coli O157 in cattle during the winter when shedding is virtually nonexistent (winter experiment) or influence cattle actively shedding E. coli O157 (summer experiment). Yearling cattle were group-penned under dry-lot conditions, adjusted to a high concentrate ration, and randomly assigned to treatment: control (1 mL corn oil injected s.c. daily) or triiodothyronine (T(3); 1.5 mg suspended in corn oil injected s.c daily). Cattle were individually processed daily for collection of fecal and blood samples. Treatment with exogenous T(3) produced a significant change in serum thyroid hormone concentrations indicative of a hyperthyroid status in both experiments. No differences (P>0.10) were observed in fecal shedding of E. coli O157 in the winter experiment. In the summer experiment, fecal shedding of E. coli O157 was decreased (P=0.05) by administration of T(3) during the treatment period (days 1-10), tended to be lower (P=0.08) during the following 7-day period of no treatment, and was lower (P=0.01) when examined across the entire experimental period. Results of this research indicate that the thyroid or its hormones may be involved in the seasonal shedding patterns of E. coli O157 in cattle.