Synthroid vs. Levothyroxine

See how Synthroid and Levothyroxine compare on approved uses, dosing, side effects, and cost.

Key Takeaways

  • Synthroid is the original brand-name version of levothyroxine sodium, and both are the exact same synthetic T4 hormone used to treat hypothyroidism.

  • Because levothyroxine has a narrow therapeutic index, staying on one consistent product, whether that is Synthroid or a single generic manufacturer, matters more than it does for most drugs.

  • Generic levothyroxine usually costs far less out of pocket than brand-name Synthroid, though insurance coverage and copays vary for both.

Synthroid vs. Levothyroxine Drug Summary

Synthroid

Synthroid is the original brand-name form of levothyroxine sodium, a synthetic version of the T4 thyroid hormone. It replaces or supplements thyroid hormone in people with hypothyroidism and is also used to suppress TSH after thyroid cancer treatment. It is taken as one tablet by mouth each morning, on an empty stomach, at a dose set by weight and lab results.

Levothyroxine

Levothyroxine sodium is the generic form of synthetic T4, chemically identical to Synthroid. It treats hypothyroidism from any cause and is also used to suppress TSH in thyroid cancer management. It comes as a tablet, capsule, or IV injection, usually taken once daily by mouth on an empty stomach.

Synthroid vs. Levothyroxine Side Effects

Synthroid

Most common
Headache (dose-dependent)
Nervousness (dose-dependent)
Arrhythmias (dose-dependent)
Tremors (dose-dependent)

Levothyroxine

Most common
Headache (dose-dependent)
Nervousness (dose-dependent)
Arrhythmias (dose-dependent)
Tremors (dose-dependent)

Synthroid vs. Levothyroxine Dosage

Synthroid

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 12.5-25 mcg once daily At the start
Maximum dose 100-200 mcg once daily If needed
Dose changes 3 increases As tolerated

Levothyroxine

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 12.5-25 mcg once daily At the start
Maximum dose 1.6 mcg/kg once daily If needed
Dose changes 2 increases As tolerated

Synthroid vs. Levothyroxine Indications

Synthroid

Synthroid treats hypothyroidism of any cause, including primary thyroid failure, pituitary or hypothalamic disease, and hypothyroidism after thyroid surgery or radioactive iodine. It is also used to suppress TSH in patients managed for thyroid cancer. It is not approved for weight loss, and using it for that purpose carries an FDA boxed warning.

  • Primary hypothyroidism (thyroid failure)

  • Secondary hypothyroidism (pituitary)

  • Tertiary hypothyroidism (hypothalamic)

  • TSH suppression in thyroid cancer

  • Not approved for weight loss (boxed warning)

Levothyroxine

Levothyroxine treats the same range of hypothyroidism, whatever the underlying cause, including primary, secondary, and tertiary disease. It is also used for TSH suppression in thyroid cancer follow-up care. Like Synthroid, it carries a boxed warning against use for weight loss.

  • Primary hypothyroidism (thyroid failure)

  • Secondary hypothyroidism (pituitary)

  • Tertiary hypothyroidism (hypothalamic)

  • TSH suppression in thyroid cancer

  • Not approved for weight loss (boxed warning)

Synthroid vs. Levothyroxine Pros and Cons

Synthroid

Synthroid is made by a single manufacturer, AbbVie, using a tightly controlled process that keeps tablet potency consistent from refill to refill. Because levothyroxine has a narrow therapeutic index, that consistency can matter for staying in a stable dose range. Most insurance plans cover Synthroid, but usually at a higher copay than generic levothyroxine.

Pros

  • One consistent manufacturer, so tablet potency stays stable refill to refill

  • Widely recognized reference product most doctors default to prescribing

  • Available in 12 precise tablet strengths for fine dose adjustments

Cons

  • Roughly $30 to $60 a month without insurance, well above generic

  • Only available as a tablet, not as a capsule or oral solution

  • Brand-only, so no lower-cost version if a plan drops coverage

Levothyroxine

Generic levothyroxine is made by multiple manufacturers, so it usually costs less than brand-name Synthroid, especially without insurance. It comes in more dosage forms, including capsules and an oral solution, than Synthroid's tablet-only lineup. Because pills from different generic makers can vary within the FDA's allowed range, pharmacists suggest staying with one manufacturer once stable.

Pros

  • Roughly $4 to $50 a month without insurance, well below brand pricing

  • Made by several manufacturers, which helps during supply shortages

  • Available as capsules or oral solution, not just tablets

Cons

  • Potency can vary slightly between generic makers, unlike single-source brand

  • Switching pharmacies can mean switching manufacturers without notice

  • Less brand recognition, which can add confusion when refilling

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