According to this [1] about 12 million people have a thyroid problem and don't know it. (maybe a bad source?)
Given there's no test for depression, and it's a pretty unreliable judgement call [2], it's a pretty reasonable claim.
[1] http://www.thyroid.org/media-main/about-hypothyroidism/ [2] http://www.medscape.com/viewarticle/706714