Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland does not produce enough thyroid hormones, slowing metabolism. It most often results from autoimmune destruction of the thyroid (Hashimoto's thyroiditis) or after thyroid surgery or radioiodine treatment. Less commonly, it can be due to iodine deficiency, congenital defects, or pituitary or hypothalamic disease.

Symptoms are often nonspecific and include fatigue, weight gain, cold intolerance, dry skin, hair loss, constipation, and slowed thinking. Depression, menstrual irregularities, muscle weakness, and a slower heart rate may occur. In severe or long-standing cases, myxedema can develop and become life-threatening.

Diagnosis relies on blood tests showing elevated TSH and low free T4 in primary hypothyroidism; subclinical hypothyroidism has elevated TSH with normal T4. Antibody testing for thyroid peroxidase supports an autoimmune cause. Imaging or pituitary studies are rarely needed unless another disorder is suspected.

Treatment is lifelong thyroid hormone replacement, usually levothyroxine. The dose is adjusted to normalize TSH, with monitoring after starting or changing therapy (typically 6-8 weeks) and then every 6-12 months. Absorption can be affected by food, calcium, or iron; follow clinician guidance on timing and interactions. Pregnancy requires higher doses and closer monitoring.

With treatment, symptoms improve and prognosis is favorable. Untreated hypothyroidism increases the risk of cardiovascular disease, high cholesterol, and cognitive or physical decline. It is more common in women and tends to increase with age; autoimmune Hashimoto's thyroiditis is the leading cause in iodine-sufficient regions. Early treatment in infancy prevents intellectual and growth problems.