I have itp a low platelet disorder which is costly to treat. If my employer sees that can they find ways to fire me. To keep insurance premiums down for the other employees.

Employment in many States absent a contract or union is at will. Your employer can fire you for any reason at any time.

That doesn't mean that an Attorney wouldn't be interested in investigating a firing due to health discrimination.

Factors are where you live, what company policy is, and what kind of company you work for.
If you are in the US, and live in an 'at will' state, and work for a private company that has no specific policy about medical conditions, then they might will find a reason to let you go that isn't medical.

Can't give Judy a greenie yet but I agree. Most all companies have potential new hires get a physical so they can avoid hiring already sick employees. If your illness drives up insurance it can hurt the company or other employees so they could terminate you unless you are a member of a protected class. Nplate is a common treatment for itp and its frequent side effects can make a danger to yourself or others in a work environment.

Many insurers will not cover pre-existing conditions which may be a factor here. But if the company is small enough and they feel you are costing them too much they can find some excuse to terminate you.

This normally is not a issue, unless your company is so small and not part of a larger over all group.