PPID / EQUINE CUSHING'S SYNDROME:


Information reported about Cushing’s Syndrome in horses has indicated that cortisol values from blood serum collected 8-10 hours apart are very informative. Normal horses have a natural diurnal cortisol rhythm greater than 30%. The UC Davis equine research group reports that in horses with Cushing’s Syndrome, the cortisol rhythm is 30% or less. Take the first sample in the a.m. and the second 8-10 hours later. We like to assay TT4 in at least one of the samples and ideally both samples. Our expected TT4 normal is at least 12.0 ng/mL. If the animal is on treatment for hypothyroidism, draw one sample before the daily dose so that the trough level can be obtained. We also find it useful to assay insulin in both samples, as insulin concentrations are often two or more times higher than normal in horses with this condition. Grain concentrate rations will elevate insulin for several hours after feeding so do not draw samples within 5 hours after grain. Hay and pasture are allowed. Obtain blood samples with minimal stress to the animal to avoid artificially elevating the cortisol level. Efforts should be made to keep the subject in its normal environment.

The most common cause of prolonged anovulation (greater than six months) in mares is Cushing’s Syndrome. A Cushing’s Screen (see BET Protocol) should be performed in these mares. If test is positive, they will not respond to the following protocols unless treated for Cushing.

Anovulatory mares have serum concentrations of progesterone that are less than 1.0 ng/mL. A progesterone assay should be performed before starting any protocol. Additional progesterone assays should be performed following completion of the protocol to detect ovulations, which may have occurred even though the mare did not show estrus. These mares demonstrating silent heat will respond to prostaglandin injections and exhibit normal estrous cycle if TT4 levels are sufficient (i.e. greater than 12.0 ng/mL).

A Total T4 assay should be performed in all anovulatory mares or any mare not exhibiting normal estrous cycles or ovarian follicular development. Response will be significantly improved when sufficient thyroxine is given orally to elevate TT4 concentrations between 12 and 25 ng/mL. The dosage should be titrated so these concentrations are present 24 hours following the dosage of thyroxine. This simply means that the blood sample should be taken before the oral dose of thyroxine is administered and not afterwards. Thyroxine need only be given once per day. Thyroxine should be continued until at least 45-50 days of pregnancy. It is imperative to monitor TT4 concentrations to insure that the proper amount of thyroxine is being administered. Too much will cause weight loss and hyperexcitability, and too little will be ineffective.