Welcome to Connect, @smyles73. You were diagnosed 3 years ago with ET and taking 1500 mg of Hydroxurea daily since then. From what you’re saying is that the blood numbers are still as elevated? I agree with @mdterp76, it’s time for a 2nd opinion with a hematologist oncologist who specializes in MPN (myeloproliferative neoplasms).

ET and several other blood conditions are associated with MPNs. These disease can change or progress over time. If your current medication isn’t working it may be time for a few more tests to confirm the original diagnosis.

This is a pretty comprehensive informational article about MPNs from Cleveland Clinic that might be helpful for you to read.
https://my.clevelandclinic.org/health/diseases/24144-myeloproliferative-neoplasms
Have you ever had a bone marrow biopsy?

I was diagnosed with ET and was referred to the Mayo Clinic in Arizona. After a bone marrow biopsy, chromosome test, and a full abdominal ultrasound, everything was normal except CALR positive, JAK2 negative, but continually elevated platelet counts. I learned my condition is NOT genetic, but rather a DNA mutation common as you age. I am now taking a low dose 500 mg hydroxyurea MWF. But it was reassuring to know everything else was normal. I continue to eat an anti-inflammatory diet and work out at my our local gym to try to stay as healthy as possible. Grateful for the otherwise good health.