Hypothyroidism
Hypothyroidism -- an underactive thyroid gland -- is one of the most common conditions an endocrinology practice manages, affecting roughly 0.3-12% of people depending on the population, with a strong female predominance, per a 2025 JAMA review.
Symptoms
Fatigue, weight gain, cold intolerance, constipation, dry skin, hair thinning, and low mood -- often gradual enough that people attribute it to aging or stress before it's diagnosed.
Causes and risk
Hashimoto's thyroiditis (autoimmune) causes up to 85% of cases. Other causes include iodine imbalance, 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; thyroid peroxidase antibodies confirm an autoimmune cause when positive but aren't required for diagnosis.
Treatment
Levothyroxine is first-line, typically dosed around 1.6 mcg/kg/day and titrated more slowly in older adults or those with heart disease, per the ATA Guidelines for the Treatment of Hypothyroidism. TSH is rechecked at 6-8 weeks after any dose change, then every 6-12 months once stable -- getting the dose right is an iterative process, not a one-time prescription.
When to seek care
Myxedema coma -- hypothermia, altered mental status, slow heart rate, slowed breathing, low blood pressure -- is a rare but life-threatening emergency needing immediate hospital care.