Hi @amyb52. Welcome to Connect. This is a great forum for meeting other members who share your similar health story.
The ‘normal’ guidelines for platelet levels can be anywhere between 150,000 to 450,000. Yours at 478,000 is a bit higher than the upper normal range. The main concern with elevated platelets is developing a thrombosis (blood clot). So your hematologist has put you on daily aspirin which can help prevent platelets from forming clots.

When to start treatment with Hydroxyurea can depend on what your risk level is for developing clots. Doctors consider three major risk factors for thrombosis in order to group ET patients into 4 risk categories. The 3 factors are: history of thrombosis, JAK2/MPL mutations, and advanced age.

The 4 risk categories are:
Very low risk ~ absence of all three risk factors.
Low risk ~ presence of JAK2/MPL mutations
Intermediate-risk ~ presence of advanced age
High-risk ~ presence of thrombosis history or presence of both JAK2/MPL mutations and advanced age.
Source:
https://www.nature.com/articles/s41408-017-0041-8
(Blood Cancer Journal)

So your doctor will take into consideration what your needs are by following your progress with taking just the aspirin for a while. If your numbers continue to rise then they may prescribe something to lower the platelet levels. Starting medications before they’re really necessary isn’t necessarily beneficial.

You’re wondering what the best websites are to learn more about your condition. Knowledge is power but as you know, the internet can lead you down some pretty scary rabbit holes. So these are my go-to, reputable sources to help new members get started. Remember not everything will apply to you.
This one is a good primer on platelets.
https://www.verywellhealth.com/normal-platelet-count-5186765
More explanations about thrombocytosis or ET from Mayo Clinic
https://www.mayoclinic.org/diseases-conditions/thrombocytosis/symptoms-causes/syc-20378315
From Medlineplus.gov
https://medlineplus.gov/genetics/condition/essential-thrombocythemia/
From: National institute of health
https://www.nhlbi.nih.gov/health/thrombocythemia-thrombocytosis
Healthgrades:
https://www.healthgrades.com/right-care/blood-conditions/essential-thrombocythemia
Very well health:
https://www.verywellhealth.com/thrombocytosis-overview-and-more-5187124
Members in this support group are more than happy to help answer any questions or concerns so don’t hesitate to tag a member by typing in their @name or by replying directly in the conversation. That way the member will get a notification that they’ve got a reply to answer.
How often do you have your blood work taken?

My hematologist says 500 but mine have gone as high as 601 but it was after i had Covid twice. So, he agreed to watch for a few more monthly labs to see if they would go back down. 30 days ago they had dropped to 554 and i get checked
Again today.. so hopefully, they will go down some more. For the past 2 years mine have fluctuated in the 400’s. Best of luck.