Thyroid disorders can have a significant impact on your attitude and temperament. Hypothyroidism tends to make people feel depressed. This is due to the fact that too little thyroid hormones decreases the “happiness” hormone, serotonin, in the brain. Do a quick mental check, Dr. Hall says. Ask yourself if feeling blue is happening all the time, if it’s becoming a common problem in your life, or if you’re taking birth control pills. “They are a major cause of fatigue because they bring the estrogen levels down to menopausal levels,” Dr. Hall says.