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@danielad

@colleenyoung - Hi Colleen, I wonder if you can point me in the right direction. I am looking for information regarding "LOW" Parathyroid Hormone-Related Peptide (PTH-rP). I found a lot of information about High PTH-rP; however, did not find anything regarding Low PTH-rP.
Here is the reason for my request:

As you may remember, I have had Melanoma and more recently, Breast Cancer. In July 2019 (right before the discovery of my breast cancer), routine labs found High levels of Calcium. My PCP tested the PTH level, which was normal, so nothing else was done. In March 2020, I was admitted to the hospital with nausea, vomiting, and hypercalcemia. The Nephrologist on staff, now my doctor, has been looking for the cause of the Hypercalcemia since that time.

Labs in mid-April showed High Calcium, but Normal PTH and Phosphorus. Labs in mid-June showed the Calcium had decreased to the “upper limit” of Normal: "10.4" (Quest range: 8.6-10.4 mg/dl). PTH and Phosphorus were Normal. However, the PTH-Related Peptide (measured for the FIRST time), was very low: "8 pg/ml" (Quest range: 14-27 pg/ml).

The Nephrologist is puzzled. Because of the low PTH-rP, 3days ago he decided to order a "Sestamibi scan". However, by the afternoon, he had changed his mind; he now wants to wait another 2 months and get new labs to see if anything changed. I really don't do "waiting" too well; so, in the meantime, I would like to understand more about the low Parathyroid Hormone-Related Peptide.

Thanks Colleen!

-- Dee

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Replies to "@colleenyoung - Hi Colleen, I wonder if you can point me in the right direction. I..."

Hi @danielad, that is a question outside of my knowledge. I couldn't find much about low levels either.

However I found this information by Mayo Clinic Laboratories:
- Parathyroid Hormone-Related Peptide, Plasma https://www.mayocliniclabs.com/test-catalog/Clinical+and+Interpretive/81774
As you noted, it is an elevated level of PTHrP that can indicate a need for further investigation for a malignancy.

Of particular note are the last two sentences
"Like all immunometric assays, PTHrP assays are susceptible to false-low results at extremely high analyte concentrations ("hooking") and to rare false-positive results due to heterophile antibody interference. Therefore, if test results are incongruent with the clinical picture, the laboratory should be contacted."

You said that this was the first time that your PTHrP was tested. Is it possible that is was a false low?