Is there a reason you want to do the TRH Stim test? We usually recommend that when you’ve had equivocal endogenous testing done before - symptoms suggest a positive test but endogenous test comes back normal. I don’t see that you’ve done that in your case history. The TRH Stim test is not recommended during the fall rise because there are no testing normals to which you can compare your results. The fall rise technically begins at the summer solstice at the end of June and lasts until the winter solstice in December. It peaks at the equinox in September. I would not spend the money to have a TRH Stim test done in August because it might well be misleading. You can do the regular endogenous test as there are fall rise values to compare to. Untreated PPID horses generally show an exaggerated fall rise, which can be diagnostic for PPID.
As you’re concerned about what happens to Shakespeare in the fall, with your vet’s agreement you could start pergolide now and note its effects or lack thereof. Then, after the rise, you could take him off the pergolide for three weeks and test, using the Stim test if that seems important to you. Waiting too late in the rise to begin pergolide has shown to be less effective at controlling PPID than starting it earlier.
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