Can my partner apply as a dependent for a medical DTV visa in Thailand?

Since you're married, the usual route is what John mentioned: you get your medical DTV first, then your spouse applies for their own DTV under the "spouse of a DTV holder" category.

On the official Thai e-Visa site (thaievisa.go.th), the spouse checklist is: passport, a recent photo, proof of residence in the country you apply from, a copy of your approved DTV, your marriage certificate, a criminal record clearance, and financial evidence of at least 500,000 THB, either the last 3 months of bank statements or a sponsorship letter. That category doesn't list a treatment plan or activity letter, so your spouse's "reason" is simply being your spouse. Each embassy adds its own details, so check the page of the one you're applying through.

Are you married??

Normally you should have a long term treatment plan signed off by a recognised medical professional at a legitimate medical facility. Most will want to examine you before they will provide this. Your spouse can apply for their own DTV as your dependant once your DTV has been issued

I did but pay agent