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.
HORMONAL VALUES EXPECTED ON NORMAL HORSES
| HORMONE | NORMAL VALUES |
|---|---|
| INSULIN | 2 - 25 µI.U./mL |
| CORTISOL (RESTING) | 20 – 90 ng/mL |
| CORTISOL (POST DEX) | Less than 10 ng/mL |
| CORTISOL RHYTHM | Greater than 30% |
| ACTH | Less than 30 pg/mL Mid Nov - Mid July. Less than 100 pg/ml Mid July to Mid Nov. |
PPID / EQUINE CUSHING’S PROTOCOLS
Diurnal Cortisol Rhythm Test
For the BET Cushing’s Screen (Diurnal Cortisol Rhythm) the following protocol is suggested:
- Two samples taken 8-10 hours apart.
- Fasting, no grain for at least 5 hours before drawing the sample (grass and hay are fine).
- Do not unduly stress the animal, i.e. normal exercise, turnout > routine.
| SAMPLE TIME | HORMONES TO ASSAY |
|---|---|
| AM | TOTAL T4, INSULIN, CORTISOL |
| PM SAMPLE (8-10 HOURS LATER) | TOTAL T4, INSULIN, CORTISOL |
| Required: Two mL of serum or plasma at each time period. | |
| Sample should be separated and sent in clean, labeled, unbreakable vial. |
Low Dose Dexamethasone Test
- We do NOT recommend using Dexamethasone on laminitis or pregnant animals.
- A normal horse will have a cortisol suppressed below 10 ng/mL.
| SAMPLE TIME | HORMONES TO ASSAY | |
|---|---|---|
| SAMPLE #1 (TAKEN AT 4-6 PM) | CORTISOL | |
| Administer 0.04 mg/kg Dexamethasone IM | ||
| (20 mg for a 1000 pound horse) | ||
| SAMPLE #1 (18-20 HOURS LATER NOON NEXT DAY) | CORTISOL | |
| Required: Two mL of serum or plasma at each time period. | ||
| Sample should be separated and sent in clean, labeled, unbreakable vial. |
Dexamethasone Suppression Tests
Same Animal
- Draw the final sample the next day around noon (18-20 hours later).
- Separate the serum, label all three samples and send them at the same time.
| SAMPLE TIME | HORMONES TO ASSAY | |
|---|---|---|
| AM (between 8-10 am, before breakfast) | TOTAL T4, INSULIN, CORTISOL | |
| PM ( 8-10 hours later before dinner) | TOTAL T4, INSULIN, CORTISOL | |
| Administer 0.04 mg/kg Dexamethasone IM | ||
| (20 mg for a 1000 pound horse) | ||
| Post DEX (18-20 hours later, around noon next day) | CORTISOL |
ACTH Test
| SAMPLE TIME | HORMONES TO ASSAY |
|---|---|
| ANYTIME | ACTH |
| **A normal horse will have an ACTH level of less than 30 pg/mL** | |
| One mL of EDTA PLASMA, HARVESTED and FROZEN, in PLASTIC VIAL sent OVERNIGHT is required for ACTH test. |