How it causes the blahs: Your thyroid makes and releases the hormones that control your metabolism. So when this gland produces too little of these hormones, you may suffer from fatigue, weight gain, plus depression itself.
The fix: Ask your doctor for a blood test to measure your levels of thyroid hormones. "If they're low, your doctor will likely prescribe a synthetic form of thyroid hormone to correct the imbalance," says Gary Ross, M.D., coauthor of Depression & Your Thyroid.
Vitamin B12 deficiency
How it causes the blahs: Your body needs vitamin B12, found mostly in animal products, to maintain a healthy nervous system. Without enough, you may suffer from fatigue, weakness, other symptoms of depression, and more. A B12 deficiency usually occurs because your body can't absorb the vitamin due to a problem in the stomach or small intestine, which can be caused by autoimmune or gastrointestinal disorders, surgery, regular use of proton pump inhibitors, like Prilosec, or a vegetarian diet.
The fix: A B12 deficiency can cause disability or death, so ask your doctor for a urinary methylmalonic acid test, says Sally Pacholok, R.N., coauthor of Could It Be B12? An Epidemic of Misdiagnoses. If diagnosed, you'll likely need lifelong B12 injections or daily doses of B12 lozenges.
WHEN IT'S REALLY SAD
If you get slammed with depression around the same time every winter, the culprit may indeed be SAD. "Darker days cause rhythm shifts in mood-altering hormones like melatonin, serotonin, and epinephrine, which can affect your sleep and energy levels," says Stephen Lurie, M.D., an assistant professor at the University of Rochester Medical Center. Talk to your doctor, who may recommend light therapy (sitting in front of a bright light box for 15 to 45 minutes daily) and/or an antidepressant, such as Wellbutrin.