Levothyroxine: Answers to the Most Frequently Asked Questions

What Is Levothyroxine Used For?

Levothyroxine (brand names Synthroid, Levoxyl, Tirosint) is a synthetic thyroid hormone used to treat hypothyroidism — a condition in which the thyroid gland does not produce enough thyroid hormone. It is the third most prescribed medication in the United States, with over 100 million prescriptions written annually.

Levothyroxine is also used to suppress TSH (thyroid-stimulating hormone) in patients with thyroid cancer after thyroidectomy, and in the treatment of goiter (enlarged thyroid).

When Should I Take Levothyroxine?

Levothyroxine should be taken on an empty stomach, 30–60 minutes before breakfast (or 3–4 hours after your last meal). Food, especially high-fiber foods, calcium, and iron, significantly reduces levothyroxine absorption. Consistent timing is critical — taking it at the same time each day, in the same way, ensures stable thyroid hormone levels.

An alternative is to take levothyroxine at bedtime, at least 3–4 hours after your last meal. Some studies suggest bedtime dosing may actually achieve slightly better TSH control than morning dosing, possibly because the stomach is more reliably empty.

What Foods and Supplements Interfere with Levothyroxine?

Several substances reduce levothyroxine absorption and should be taken at least 4 hours apart:

Calcium supplements (including calcium-fortified foods like orange juice)
Iron supplements (ferrous sulfate, ferrous gluconate)
Antacids containing calcium or aluminum (Tums, Maalox)
Proton pump inhibitors (omeprazole, pantoprazole) — reduce stomach acid needed for absorption
Soy products (soy milk, tofu) — can reduce absorption
High-fiber foods (bran, fiber supplements) — reduce absorption
Coffee — even black coffee reduces levothyroxine absorption; wait 30–60 minutes after taking levothyroxine before having coffee

Why Does My Levothyroxine Dose Keep Changing?

Levothyroxine requirements change over time due to several factors:

Weight changes: Levothyroxine dosing is weight-based (~1.6 mcg/kg/day); significant weight gain or loss requires dose adjustment.
Pregnancy: Thyroid hormone requirements increase by 25–50% during pregnancy — TSH should be checked immediately upon confirmation of pregnancy.
Aging: Older adults often require lower doses due to reduced metabolic rate.
Medications: New medications that affect absorption or metabolism may require dose adjustment.
Absorption issues: GI conditions (celiac disease, bariatric surgery) can reduce absorption.

Is Generic Levothyroxine as Good as Synthroid?

This is one of the most debated questions in thyroid medicine. The FDA considers generic levothyroxine bioequivalent to Synthroid, and large studies have not found meaningful clinical differences between them. However, levothyroxine has a narrow therapeutic index — small differences in absorption can affect TSH levels. The key principle is consistency: do not switch between brands or between brand and generic without rechecking TSH 6–8 weeks later. If you are stable on a particular formulation, stay with it.

What Are the Signs That My Dose Is Too High or Too Low?

Signs of too little levothyroxine (undertreated hypothyroidism): Fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, brain fog, depression, elevated TSH.

Signs of too much levothyroxine (overtreatment): Palpitations, tremor, anxiety, insomnia, heat intolerance, weight loss, diarrhea, increased sweating, low TSH (suppressed).

How Much Does Levothyroxine Cost Without Insurance?

Generic levothyroxine is very affordable — typically $10–$20 for a 30-day supply using GoodRx or RxGo coupons. Brand-name Synthroid costs $50–$100/month without insurance. Tirosint (a liquid gel cap formulation with fewer inactive ingredients, preferred for patients with absorption issues) costs $150–$200/month.

References

Jonklaas J, et al. Guidelines for the treatment of hypothyroidism. Thyroid. 2014;24(12):1670-1751.
American Thyroid Association. Hypothyroidism Booklet. 2020.