What other options do I have apart from hospitalization insurance?

Based on my experience, most insurers will allow you to upgrade your current hospitalisation plan / purchase riders but with an exclusion clause for thyroid & its related health issues. Once you are fully discharged by your doctor, you may try to appeal with your insurer to remove the exclusion (though I was advised by my agent that the success of such appeal is very low). However, if you decided to stick with your current plan, you will still be able to claim for Hyperthyroidism and Atrial Fibrillation.

If you hold a full time job at the moment, do look at your company insurance as most corporate insurance covers pre-existing / existing / new health condition. PS: I have Grave's disease too and am currently claiming from my company's insurance for my regular outpatient visits (blood test, consultation and medications).

(non-finance related) Lastly, would recommend you to:

1. drink green tea daily! Doing so has shown great effects in helping me to lower my resting heart rate from 115-120 to 90-100
2. include one spoon of extra virgin olive oil in your daily diet as it has been shown in many reports to help protect your heart
3. get a fitness tracker so that you can track your heartrate

try consulting with an agent. If results normal, no relapse, maybe get a medical report that you are all ok, you might be able to apply.