It's very complicated!
Some illnesses have symptoms that are subsets of others but you often can't treat them like the illness it emulates (at least partially).
POIS has symptoms of depression, yet it's likely not the best that a psychiatrist treats POIS. However a patient can have POIS AND depression as diseases in their own right. This has really complicated POIS.
Some of us only have depressive symptoms while in POIS, and they completely disappear outside of POIS. Ot would seem that such a person is not normally depressive. A psychiatrist is not able to make the distinction, for all that you tell him about POIS, because POIS does not officially exist, nevermind have a known form of treatment.
We have had many members who have suffered greatly on anti-depressants, because they were not designed for POIS, but none of us, and in reality there is no-one who can officially recommend what to offer as a solution.
We are desperately working to find a solution, at least the tip of the iceberg as far as a remedy goes. But until then, we are all in the same boat.
Check out the various helpful remedies that some have found. None seem to work for everyone, but maybe one or two may help you.