Overview

Synthroid is a brand of levothyroxine sodium, a synthetic version of the human thyroid hormone T4. When the thyroid gland does not make enough hormone, Synthroid restores normal levels and helps regulate metabolism, energy, heart function, and growth.

Areas of relevance: thyroid disease, oncology support for thyroid cancer protocols, hormone replacement, and fitness medicine where appropriate medical evaluation confirms hypothyroidism. It is not indicated as a weight loss drug or performance enhancer.

Mechanism in brief: levothyroxine (T4) is converted in tissues to triiodothyronine (T3), which binds nuclear receptors to influence gene transcription and metabolic rate.

Who may be prescribed Synthroid

Clinicians may prescribe Synthroid for:

  • Primary, secondary, or tertiary hypothyroidism of any cause, including autoimmune thyroiditis.
  • Management of certain goiters and to prevent goiter recurrence after surgery or radioiodine.
  • Adjunct treatment in specific thyroid cancers, typically to suppress TSH after surgery and radioiodine as directed by an oncologist.

It replaces or supplements thyroid hormone when the body cannot produce adequate amounts on its own.

Strengths and ingredients

Active ingredient: levothyroxine sodium (T4).

Available tablet strengths commonly dispensed in the USA include: 200 mcg, 150 mcg, 125 mcg, 100 mcg, 75 mcg, 50 mcg, and 25 mcg. Color and excipients vary by strength and manufacturer.

Only your prescriber can set the correct dose based on TSH and free T4 levels, age, weight, cardiac status, and pregnancy status.

How to take Synthroid

General directions. Follow your prescribers instructions exactly.

  • Take it once daily at the same time to keep levels steady.
  • Swallow on an empty stomach with a full glass of water, 30 to 60 minutes before breakfast.
  • Avoid taking calcium, iron, antacids, or bile acid sequestrants for at least 4 hours after your dose because they reduce absorption.
  • Continue therapy even if you feel well; thyroid replacement is typically long term.
  • If you miss a dose, take it when you remember. If it is close to the next dose, skip the missed dose. Do not double up.
  • For infants or patients unable to swallow: crush the prescribed tablet and mix with 1 to 2 teaspoons (5 to 10 mL) of water only, then administer immediately. Do not store the mixture. Do not mix with soybean infant formula.
  • Do not change brands or doses unless your clinician advises; switching products may require repeat labs.
  • Symptom improvement may take several weeks while your dose is optimized.

If you have questions about timing with meals, supplements, or other medicines, contact your pharmacist or prescriber. Return to top.

Storage

Keep tablets in the original container at 68 to 77 F (20 to 25 C). Short excursions to 59 to 86 F (15 to 30 C) are acceptable. Protect from heat, light, and moisture. Do not store in a bathroom. Keep out of reach of children and pets.

Safety warnings

Do not use Synthroid if any of the following apply

  • You have untreated adrenal insufficiency.
  • You are allergic to levothyroxine or any tablet ingredient.
  • You have uncorrected thyrotoxicosis or have had a recent heart attack.

Tell your doctor before use

  • If you have heart disease, history of arrhythmias, chest pain, or high blood pressure.
  • If you are pregnant, planning pregnancy, or breastfeeding.
  • If you have diabetes, bleeding disorders, osteoporosis, pituitary or adrenal problems, or other thyroid disorders.
  • If you have difficulty swallowing or known issues absorbing nutrients from the gut.
  • If you recently had surgery or you have an upcoming procedure.
  • If you take any prescription or nonprescription drugs, vitamins, minerals, or herbal products.
  • If you have allergies to foods, dyes, or other substances.

Additional important advice

  • Take only as prescribed. Too much thyroid hormone can strain the heart and bones.
  • Inform all clinicians and your dentist that you take levothyroxine before any procedure.
  • People with diabetes may see changes in blood glucose; monitor and ask whether your diabetes medicines need adjustments.
  • Foods rich in soy, high fiber diets, walnuts, and cottonseed meal can lower absorption; consistent eating patterns help your prescriber adjust your dose.
  • Mild hair shedding can occur in the first months and is usually temporary.
  • Regular lab monitoring of TSH and sometimes free T4 is needed. Children and teens may require growth checks.
  • Older adults may require lower starting doses to reduce cardiac risk.
  • Pregnancy often increases levothyroxine needs; contact your clinician promptly if you become pregnant.

Interactions

Many products change levothyroxine absorption or action. Discuss all medicines and supplements with your prescriber or pharmacist.

  • Minerals and antacids: calcium carbonate, iron salts, aluminum or magnesium antacids, sucralfate, and cholestyramine or colesevelam reduce absorption. Separate by at least 4 hours.
  • Drugs affecting thyroid hormone metabolism or binding: estrogens, testosterone, growth hormone, sertraline, carbamazepine, phenytoin, phenobarbital, rifampin.
  • Diabetes medicines and warfarin: thyroid hormone can change insulin requirements and affect anticoagulation response; closer monitoring is advised.
  • Proton pump inhibitors and H2 blockers: may subtly alter absorption; consistent timing helps.
  • Food and beverages: soy-containing products, high fiber meals, and espresso coffee close to dosing may lower absorption.
  • Other thyroid therapies or weight loss agents: do not combine with liothyronine or sympathomimetics unless prescribed and supervised.

This is not a complete list. Check before starting, stopping, or changing any medicine. See also how to take it regarding spacing from supplements.

Possible side effects

Most people tolerate levothyroxine well when dosed correctly. Symptoms of overtreatment or sensitivity can occur.

Seek medical attention urgently for signs of severe reaction or overdose, including:

  • Rash, hives, itching, swelling of face, lips, mouth, or tongue, trouble breathing, or sudden hoarseness.
  • Chest pain, fast or irregular heartbeat, shortness of breath, fainting.
  • Severe headache, tremor, nervousness, anxiety, insomnia, heat intolerance, sweating, fever.
  • Marked changes in appetite or weight, vomiting, diarrhea, stomach cramps.
  • Menstrual changes, leg cramps, joint pain, muscle weakness, seizures.

Mild temporary hair loss may occur at the start of therapy. Report persistent or severe effects to your clinician.

Alternatives and price comparison in USD

Costs vary by dose, pharmacy, insurance, and discounts. Approximate monthly cash prices in the USA for common adult doses are below. These are rough estimates to help you compare options.

  • Levothyroxine generic tablets (various manufacturers): about $3 to $15 with common coupons, $10 to $25 typical retail without discounts.
  • Synthroid brand tablets: about $20 to $50; some doses may be higher at smaller pharmacies.
  • Levoxyl brand tablets: about $15 to $40.
  • Tirosint softgel: about $90 to $150; Tirosint-SOL oral solution may be similar or slightly higher. Softgels or solution can help when absorption is inconsistent.
  • Unithroid brand tablets: about $20 to $45.
  • Armour Thyroid or NP Thyroid (desiccated thyroid, T4+T3): about $20 to $60 depending on strength; note different active profile and not directly interchangeable with levothyroxine.
  • Liothyronine (Cytomel, T3): about $10 to $40; usually reserved for specific cases or combination therapy under specialist care.

Therapeutic alternatives:

  • Stay on one levothyroxine brand or manufacturer if possible to keep levels stable; if changed, recheck TSH in 6 to 8 weeks.
  • Tirosint or Tirosint-SOL may be considered for patients with celiac disease, atrophic gastritis, PPI use, or suspected excipient sensitivity.
  • Combination therapy with liothyronine is sometimes used by specialists when symptoms persist despite normal TSH, but evidence is mixed and dosing requires care.
  • Desiccated thyroid extract is available but has variable T3 content and is not first line in most guidelines.

Ask your clinician which option aligns with your diagnosis, absorption needs, and budget. Return to top.