TRH Response Tests When On Pergolide


Eleanor Kellon, VMD
 

More vets seem to be doing TRH Response tests to check pergolide dosing. If they normalize, great - but apparently that's not often the case even when ACTH at baseline is well controlled.

The Liphook Equine Hospital group/Andy Durham in the UK did a study on TRH Response tests sent to their laboratory before and after pergolide. Very few normalized but most improved in terms of the size of the ACTH jump after TRH was given. However, that study had no information on pergolide dose of symptoms/signs. The study we are running currently is looking at those things too so hopefully will give us some more information but until the data is all in and evaluated we still can't tell you what to expect.

PPID is more complicated than ACTH levels. All of the other POMC hormones from the intermediate lobe - beta-endorphin and alpha-melanocyte stimulating hormone, beta-LPH as well as ACTH precursors like CLIP - can also influence insulin sensitivity. Prolactin does not come from the intermediate lobe but it is also controlled by dopamine. Prolactin is involved in shedding, udder enlargement and possibly the reproductive abnormalities. Dopamine deficiency in the hypothalamus, where all these dopamine producing neurons originate, is likely behind the temperature regulation/sweating abnormalities that PPID horses may show - both anhidrosis and hyperhidrosis.

Here is the first report linking functioning pituitary tissue to Cushing's syndrome in horses https://pubmed.ncbi.nlm.nih.gov/220013/ . This was the first study showing ACTH is just the tip of the iceberg https://pubmed.ncbi.nlm.nih.gov/6276164/ .

Despite all these complicated interactions, the only tests we have available to us are ACTH and TRH stimulation of ACTH. That's strike one.  TRH stimulation is more sensitive than just baseline ACTH but it can have false positives and we really don't know what a positive response means.  There could be hypertrophied intermediate lobes that are not putting out any active hormones unless stimulated by the TRH. There could be hypertrophied intermediate lobes putting out active hormones but at low and insignificant levels unless stimulated. For now, even using the TRH Response test should be based on the presence of observable signs, not age or curiosity alone. It may be that long term use of appropriate levels of pergolide can actually shrink these growths and that would coincide with normalization of the TRH Response test.  We just don't know. Also don't know the significance of normalized baseline ACTH but still abnormal TRH Response tests. Is that adequate control or not?  The presence or absence of clinical signs may be the answer.
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Eleanor in PA

www.drkellon.com 
EC Owner 2001


 

It's possible that this has something to do with the tests. https://idppid.com The qualifications excludes horses on Prascend but allows horses on CP.
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Cass, Sonoma Co., CA 2012
ECIR Group Moderator
Cayuse and Diamond Case History Folder                
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Eleanor Kellon, VMD
 

Cass,

This is what gives me pause about putting too much emphasis on TRH Response when baseline ACTH is normal.

It is sponsored by the manufacturer of Prascend.  If Prascend is superior to compounded pergolide, why exclude horses on Prascend?  Shouldn't they show a superior result? Pretty transparent.
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Eleanor in PA

www.drkellon.com 
EC Owner 2001


Nancy & Vinnie & Summer
 

This is probaby a dumb question, but I will ask anyway.  Has there been any study or luck using medicinal mushrooms on helping the pituitary tumor and could this have a positive effect on PPID symptoms?

Side note, this is also what puzzles me about CBD because of the perceived positive effects on melanoma etc, but the risk of using it alongside pergolide because of the dopamine component?

Thanks Nan
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Nancy and Vinnie and Summer
Oakley, Ca
Joined Nov 2018
https://ecir.groups.io/g/CaseHistory/files/Nancy%20and%20Vinnie 
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Summer
https://ecir.groups.io/g/CaseHistory/files/Nancy%20and%20Vinnie/Summer 
https://ecir.groups.io/g/CaseHistory/album?id=249104


Eleanor Kellon, VMD
 
Edited

There are no dumb questions and I will research your question on pituitary and mushrooms and get back to you later but on the CBD and pergolide issue, CBD blocks the effect of dopamine on dopamine receptors while pergolide substitutes for dopamine. Insufficient dopamine is the issue in PPID.  Pergolide works by substituting for it while CBD will block that effect.
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Eleanor in PA

www.drkellon.com 
EC Owner 2001


Nancy & Vinnie & Summer