Hypothyroidism
Hypothyroidism is an underactive thyroid gland, affecting roughly 0.3-12% of people depending on the population studied, with a strong female predominance, per a 2025 JAMA review.
Symptoms
Fatigue, weight gain, cold intolerance, constipation, dry skin, hair thinning, and low mood are typical, though symptoms are often gradual and can overlap with many other conditions.
Causes and risk
Hashimoto's thyroiditis (autoimmune) causes up to 85% of cases. Other causes include iodine deficiency or excess, medications (amiodarone, lithium, checkpoint inhibitors), prior thyroid surgery or radiation, and postpartum thyroiditis.
How it is evaluated
TSH is the first-line test, with reflex free T4 if abnormal -- there is no role for routine T3 testing, per AAFP diagnostic guidance. Thyroid peroxidase antibodies confirm an autoimmune cause when positive but are not required for diagnosis. Routine screening of asymptomatic adults is not recommended.
Treatment
Levothyroxine, dosed at roughly 1.6mcg/kg/day (lower for older adults or those with cardiovascular disease), is first-line, with TSH rechecked at 6-8 weeks and then annually once stable, per the 2025 JAMA review.
When to seek care
Severe, untreated hypothyroidism can progress to myxedema coma -- altered mental status, low body temperature, a slow heart rate, and low blood pressure -- an ICU-level emergency with 25-70% mortality, per current JAMA review data. This requires immediate hospital care, not oral medication alone.