TRH Stimulation Test

EDTA plasma

Currently regarded as the most accurate diagnostic test for PPID in
horses. TRH promptly stimulates release of ACTH from melanotrope
cells in the pars intermedia.
Protocol:

1. Collect 1st EDTA plasma sample for measurement of baseline
   ACTH concentration
2. Inject 1 mg TRH IV (or 0.5 mg for a pony)
3. Exactly 10 minutes later, collect 2nd EDTA plasma sample for
   measurement of stimulated ACTH concentration (test can also
   be performed with a 30 minute sample).

Interpretation:
ACTH response to TRH is seasonally affected.

January to June :
 normal horses have ACTH concentrations <110 pg/mL  PPID cases have ACTH concentrations >200 pg/mL
 Horses with intermediate results (111-199 pg/mL) are
considered equivocal/mild PPID cases and should be monitored
further clinically and retested again after 3-12 months
depending on the degree of clinical concern

July to December
The secretory response to TRH is much greater in the late
summer/autumn than at other times of year. Although some data is
available (see table), caution should still be exercised when diagnosing

PPID from TRH stimulation during this period. However, a negative
response (<110 pg/mL) at this time is reliable in ruling out PPID.

Assessment of insulin dysregulation is an important part of endocrine
investigations and might be warranted alongside TRH stimulation
testing. However, Karo Light syrup testing affects ACTH response to
TRH and therefore the TRH stimulation test should not be performed
following Karo Light syrup administration.

Time of Year

Sampling

Time post-
TRH

Hart et al 2026* Adams et al 2023#
PPID
Unlikely
Interpretive
Zone (consider
clinical signs &
signalment)
PPID
Likely
Cutoff (based on
Youden’s index)

January – June

10 min < 100 100 – 200 > 200 108
30 min < 40 40 – 90 > 90 55.8

July – December

10 min < 100 TRH stimulation testing is most
useful to identify negative
cases in these months.

434
30 min < 40 209

See also:
ACTH
Insulin
PPID