Observations on thyroid hormones in the blood of thoroughbreds

During an investigation of a group of clinically well thoroughbreds housed in the same stable, it was noted that seven of the 14 had no detectable thyroxine in the plasma (less than 5 nmol/litre). An investigation of thoroughbreds in this stable over a five month period suggested that the thyroids were functioning normally and that the thyroxine was excreted in the urine. Thyroxine binding studies on the blood of these horses suggested that the major part of the circulating thyroxine was bound to albumin. The thyroxine had been displaced resulting in an apparent absence of plasma thyroxine since the limit of detection for normal radioimmunoassay is 5 nmol/litre. It is concluded that the measurement of plasma thyroxine as an indication of thyroid function in the thoroughbred can prove unreliable.