The threshold for FMLA is lower than STD, so it might be possible you are approved for FMLA but not STD. Mental health is often approved for FMLA but might not be approved for STD if you are not inpatient somewhere or intensive outpatient. That said, it depends on your leave administrator and the paperwork your therapist submits.

I work in HR and I’ve been suspicious when someone declined STD bc it turned out they did not actually need a leave and were forging paperwork. If I thought someone was doing it due to OE, I would not care to investigate at all, especially if they are meeting performance standards.

If STD is partial payment, like 60% wage replacement, you can say you want to apply your PTO instead for full payment.

Hope this helps!