One always imagines that after the diagnosis of an under active thyroid and it's treatment, that the patient will have lost weight, be "up and about" bright eyed and bushy tailed, but this doesn't happen. The improvement is gradual taking over months than weeks to notice any improvement. Depression and under active thyroid are difficult to differentiate on clinical grounds and may even be co-existent.
Do you mean he has been treated for thyroid problems and the blood tests are all clear, or do you mean that blood tests for an underactive thyroid were normal? I am a bit confused.