Thyroid function in mature horses ingesting endophyte-infected fescue seed

OBJECTIVE: To determine whether ingestion of fescue seed infected with the endophyte Neotyphodium coenophialum would alter thyroid function in adult horses. DESIGN: Original study. ANIMALS: 4 adult mares that were not pregnant and 6 adult geldings. PROCEDURE: Thyrotropin releasing hormone stimulation tests were performed while horses received a standard diet and after infected seed (2.3 kg/d 5 lb/d]) had been fed for 1 and 2 months. Serum prolactin concentrations were measured to verify endophyte absorption. RESULTS: Serum prolactin concentrations indicated that at least 8 of 10 horses absorbed the endophyte. Baseline concentrations of thyroid stimulating hormone, total and free triiodothyronine, and total and free thyroxine and the change in hormone concentrations in response to administration of thyrotropin releasing hormone (1 mg, i.v.) were not altered by ingestion of endophyte-infected fescue seed. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that ingestion of fescue seed infected with the endophytic fungus N. coenophialum for 2 months has little effect on thyroid function in adult horses that are not pregnant.