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
Levothyroxine
Synthroid vs. Levothyroxine Dosage
Synthroid
Levothyroxine
| 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 |
| 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
| 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
| 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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