Although they're separate diseases, depression is sometimes a symptom of hypothyroidism, when your thyroid gland doesn't make enough thyroid hormone. Medication can boost those levels, and that can improve or get rid of your symptoms, including depression.
The two conditions share so many signs -- fatigue, sluggishness, trouble concentrating, sleeping too much -- that doctors sometimes overlook the possibility that someone who is depressed may also have low thyroid levels. Blood tests can help your doctor diagnose hypothyroidism.