Hypothyroidism

What it means

Thyroid hormone helps set how fast the body uses energy. When levels are low, symptoms often develop slowly and can include tiredness, weight gain, feeling cold, dry skin, thinning hair, constipation, a slow heart rate, heavy or irregular periods and low mood.

The NIDDK notes that fatigue and weight gain are common and do not by themselves mean a thyroid problem. Diagnosis needs blood tests, usually starting with TSH, a hormone from the pituitary that rises when the thyroid is underactive.

Treatment replaces the missing hormone, usually with a daily pill. Blood tests are repeated after starting and after dose changes until the level is stable.

Untreated hypothyroidism may add a modest amount of weight, often from fluid as well as fat. Treatment can help, but it rarely explains or reverses large amounts of weight gain on its own. A thyroid check is a common step when weight changes are unexplained.

Why it matters

Checking thyroid function rules out a treatable cause of weight gain before or during weight-loss treatment.