Usually aging metabolism, less movement, poor sleep, or a medication - but weight gain with fatigue, cold intolerance, and constipation deserves a thyroid blood test.
What to do
Common questions
Why am I gaining weight without eating more?
Gaining weight while eating the same means the other side of the equation moved - or something medical is driving it. Start with the quiet drivers: calorie needs drop with age as muscle mass falls, and activity drifts down without anyone deciding it did. Sleep loss and stress push weight up through hormones and appetite. And medications are a major, missed cause - MedlinePlus and clinical references point to antidepressants, some diabetes drugs, steroids, beta blockers, and some birth control. The medical causes worth testing: an underactive thyroid is the classic - weight gain with fatigue, feeling cold, constipation, dry skin, and a slow heart rate, and it takes one blood test to rule in or out. Fluid retention from heart, kidney, or liver problems shows up as fast gain with swollen ankles. In women, PCOS pairs gain with irregular periods and acne, and menopause shifts where fat lands. Worth doing before the appointment: weigh yourself on the same scale weekly, note the pace - two pounds a year is aging; five pounds in a month without explanation deserves a visit. Bring your medication list and family thyroid history. Pymander is a free AI doctor, 24/7 by text. Describe the pace and the company it keeps, and it can help you decide what to ask for.