To diagnose PPID, veterinarians can perform a clinical examination, take blood tests or use MRI scans.
Initially a diagnosis will be made by observing the clinical symptoms. In particular, hypertrichosis gives a clear indication, although hypertrichosis occurs at an advanced stage of the disease. To diagnose the syndrome in an earlier stage is more difficult as the clinical symptoms are less clear. Unfortunately, this often hinders early intervention.
Besides clinical diagnosis the veterinarian has the following tests at his disposal:
- ACTH test
- Dexamethasone-suppression test (DST)
- TRH stimulation test
- Domperidone response test
- Glucose and insulin concentration test
This test determines the quantity of ACTH in the blood. It should be noted that ACTH produced in the intermediate lobe of the pituitary gland is biologically less active than ACTH produced by the anterior lobe of a healthy pituitary gland. Therefore the test shows the quantity of ACTH but not its effectiveness. Some horses with high ACTH values do not develop ppid.
Another available test to determine the presence of ppid is the dexamethasone suppression test (DST). Dexamethasone is a synthetic version of cortisol. Administering this medicine makes is possible to determine if the adrenal glands produce less cortisol. In a healthy horse that will be the case, but not in a PPID horse.
TRH stimulation test
Some cells in the intermediate lobe of the pituitary gland secrete ACTH under the influence of thyrothropine-releasing hormone (thyrotropin releasing hormone/TRH). When administering TRH PPID horses will produce significantly more ACTH than healthy horses. This test can be used in case the ACTH test as well as the dst give questionable results. This test is suitable to detect ppid in an early stage.
Domperidone response test
Domperidone has an inhibitory effect on dopamine. When administered, the pituitary gland of PPID horses will produce much more ACTH. In healthy horses this will hardly ever be the case.
In addition to the above tests, the veterinarian may decide to take a glucose tolerance test or an insulin reading. He would do this to confirm the common clinical phenomenon of insulin resistance.
Measuring cortisol levels in blood, saliva or urine is not sufficient, because not all ppid horses have too much cortisol in their bodies. Furthermore, a variety of other and diseases can affect cortisol levels, such as:
- Other diseases
- Heavy exercise
- Use of a twitch
Previously, an ACTH stimulation test was also used to detect abnormal cortisol levels in the blood throughout the day. Neither of these types of cortisol tests are used very often anymore.
An MRI-scan can be used to visualize the enlargement of the pituitary gland. This diagnostic tool is only needed in rare cases. Usually evaluation of clinical symptoms combined with one or more blood tests will be sufficient to diagnose PPID.