ANSWER:

This is prohibited. You must not compromise your hijab for anyone. If he asks you to dress in a 2 piece bikini would you? 

You can take vitamin supplements as there are many types of pills and injections to compensate for such deficiency.

I’m wearing full proper hijab and abaya from last 3 years, recently I’m suffering from very severe vitamin d deficiency and hairfall, and fungal infection due to sweat, as I live in tropical country. My doctor is asking me to remove hijab and abaya because it may be the cause, I don’t know what to do