Levothyroxine (Synthroid, Levoxyl)

What it's used for

Hypothyroidism (underactive thyroid), whether caused by Hashimoto's disease, surgical removal of the thyroid, or radioactive iodine treatment. Also used for TSH suppression after thyroid cancer surgery.

Typical dosing (adults)

• Starting: 25–50 mcg daily (lower in elderly or those with heart disease); full replacement is often 1.6 mcg/kg/day.
• Common range: 50–200 mcg daily.
• Take on an empty stomach, 30–60 minutes before breakfast, with a full glass of water.
• Dose adjusted every 6–8 weeks based on TSH blood tests.

Key interactions

• Calcium and iron supplements: major — block absorption significantly; separate by at least 4 hours.
• Antacids / proton pump inhibitors (omeprazole, pantoprazole): moderate — reduced absorption due to altered stomach acid.
• Coffee: moderate — reduces absorption; wait at least 30 minutes after taking the tablet.
• Warfarin: moderate — levothyroxine can increase warfarin's blood-thinning effect.
• Soy products and high-fiber foods: minor — can reduce absorption if taken at the same time.

Common side effects

When properly dosed, side effects are uncommon. Signs of too much (over-replacement): rapid heartbeat, palpitations, tremor, weight loss, anxiety, insomnia, diarrhea. Signs of too little (under-replacement): fatigue, weight gain, cold intolerance, constipation. Hair thinning can occur temporarily when starting treatment.

Who should be cautious

• Heart disease or angina (start at a low dose and increase slowly).
• Adrenal insufficiency (must be treated before starting levothyroxine).
• Elderly patients (start low, go slow).
• Do not use for weight loss — dangerous at high doses.

What to ask your pharmacist

• How long after my thyroid pill can I take my calcium or iron supplement?
• Is it OK to switch between brand-name and generic levothyroxine?
• My TSH levels keep changing — could another medication be interfering?
• Can I take my levothyroxine at bedtime instead of morning?