"4. You May Be Gaining Weight Because of a Medical Condition
The most common medical condition that causes weight gain is hypothyroidism. A deficiency of thyroid hormone can decrease metabolism, causing appetite loss and weight gain.
If you are feeling fatigued, lethargic, swelling, hoarse voice, intolerance to cold, sleeping too much, or headaches, you should see your doctor for an easy test to determine if you have hypothyroidism.
Much rarer is a condition known as Cushing's syndrome -- a disorder caused by an excess of the hormone cortisol -- that can also result in weight gain."