Hypothyroidism means an underactive thyroid: the thyroid makes too little hormone, so the body slows down. Typical signs are fatigue, weight gain, feeling cold, dry skin, hair thinning, constipation, and brain fog.
The most common cause is the autoimmune condition Hashimoto's thyroiditis. It's diagnosed with a TSH blood test and treated effectively with daily levothyroxine. Because its symptoms mirror menopause, testing helps tell them apart.