South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
no signs of laminitis were induced in this group of older horses during performance of 216
ODSTs.
Response of ACTH to TRH (Tier 2). TRH is a releasing hormone for several pituitary hormones.
Nearly 30 years ago, administration of TRH was shown to increase plasma cortisol concentration
when administered to horses and ponies with PPID. More recently, administration of TRH has
also been demonstrated to result in greater increases in plasma ACTH concentration in PPIDaffected equids than in normal aged equids (Figure 3). Because melanotropes in the PI have TRH
receptors while corticotrophs in the pars distalis do not, the increase in ACTH following TRH
administration can be attributed solely to release of ACTH and ACTH-like peptides from the pars
intermedia. This difference has led to renewed interest in using the TRH stimulation test to
support a diagnosis of PPID, especially when basal plasma ACTH concentration or results of an
ODST are equivocal. The test is performed by measuring plasma ACTH concentration before and
10 minutes after administration of 1 mg of TRH IV and a positive result is an increase in ACTH
above 110 pg/mL. Although the TRH stimulation test is currently being advocated as a “more
sensitive” test for detection of PPID in the earlier stages of the disease, the true value of this test
remains uncertain and needs to be assessed in a larger group of equids. A further reason that this
test may be pursued, rather than an ODST, would be to alleviate owner concerns about possible
exacerbation of laminitis following dexamethasone administration. As with the Tier 1 tests, the
increase in plasma ACTH concentration after TRH administration is also greater in fall months
and the ACTH threshold value, above which supports a diagnosis of PPID, has yet to be
established.
Figure 3. Median plasma ACTH
responses to 1 mg TRH IV in 44 horses
with PI hyperplasia (PH+, open circles
and solid line) and 22 horses with
normal pituitary glands (PH-, + and
dashed line) (from Durham AE et al.
Equine Vet Educ 2014;26:216).
Serum insulin concentration. Measurement of basal insulin concentration may be of benefit in
initial evaluation of equids with suspected PPID, not because insulin concentration is either
sensitive or specific for diagnosis of PPID, but because it may offer prognostic information.
Specifically, one case series found poorer long-term survival in PPID-affected equids with
hyperinsulinemia as compared to PPID equids with a normal insulin concentration. This makes
sense because insulin dysregulation has been associated with laminitis and it is also logical that
prognosis may be poorer with multiple endocrine abnormalities than with dysfunction of the
hypothalamic-pituitary-adrenal axis alone.
Treatment: Management of equids with pituitary pars intermedia dysfunction (PPID) consists of
improved husbandry, including proper nutrition and limiting competition for feed, body-clipping,
preventive health care, dentistry, and appropriate treatment of concurrent medical problems. In
addition, specific treatment with the dopamine agonist pergolide can improve quality of life and
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