Work intolerance in a horse with thyroid carcinoma

A thyroid carcinoma was diagnosed in a 14-year-old competitive trail horse with a 3-month history of work intolerance. Abnormal findings included low base-line triiodothyronine (T3) and thyroxine (T4) values, a large thyroid gland and decreased work tolerance. Nuclear medicine scanning revealed displacement of the right thyroid gland by a mass. Needle biopsy of the mass revealed neoplastic changes compatible with thyroid carcinoma. After removing the tumor surgically, T3 and T4 values returned to normal. Subsequently, the horse was able to compete successfully. Horses with work intolerance combined with a large thyroid gland may develop a thyroid carcinoma. In such horses, T3 and T4 values should be determined; if abnormal values are detected, needle biopsy and scintigraphic evaluation should be considered.