Hi Robin. Welcome to Mayo Connect. I’m trying to recollect a member who has both MDS and MPN with (ET). Hopefully they’ll pop into the conversation.
Ring sideroblasts always make me think of a fast food order.☺️ However, basically those little gems are culprits producing anemia in conditions such as MDS. The anemia may likely be causing your aches and fatigue.
I found this little informational article from Biology Insights that has a good explanation of Ring sideroblasts. You might find it interesting.
https://biologyinsights.com/ring-sideroblasts-formation-and-associated-conditions/
The hydroxyurea you’re taking, is helping to keep the excess platelet production under control. It may not necessarily help with the anemia conditions. That’s such a miserable feeling to be so sluggish. How are your hemoglobin level and ferritin levels?
To answer one concern you have, yes, with blood conditions the lab results may fluctuate from one test to another. It can be startling to see how quickly blood cells are impacted. When I was having blood work daily for a period of about 2 months and then every few days for months, it was astounding but fascinating to see how rapidly numbers changed. Because there are fluctuations in lab results, doctors tend to look at overall trends to determine the significance.

Have you talked to your doctor about the change in your symptoms? Are you working with a hematologist who specializes in MDS or MPNs?