Is inability to concentrate usually a symptom of depression?

Possibly: Difficulty concentrating can indeed be one of the symptoms of depression, but other things can cause this also. Such as, anxiety disorders, attention deficit disorders, and some psychotic disorders. Substance abuse and/or dependence can lead to difficulty concentrating as well. Best bet would be to consult a psychiatrist for thorough evaluation.

Could be thyroid: It may be depression, and if you have reason to think you're depressed, you're probably right. But, poor concentration can also be a symptom of thyroid disease (both too much and too little thyroid hormone) so you might want to discuss this with your primary care doctor.