Synthroid vs. Unithroid

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

Key Takeaways

  • Synthroid and Unithroid are both brand-name levothyroxine sodium tablets, so they treat the same conditions with the same mechanism, dosing logic, and monitoring requirements.

  • The two products mainly differ in manufacturer, a couple of inactive tablet ingredients such as talc, and which pharmacies tend to stock as their default brand.

  • Neither drug is approved for weight loss, and taking extra doses for that purpose can cause serious heart rhythm problems and bone loss over time.

Synthroid vs. Unithroid Drug Summary

Synthroid

Synthroid is a brand-name synthetic thyroid hormone (levothyroxine sodium) used to treat hypothyroidism and to suppress TSH after thyroid cancer surgery. It's taken as one tablet by mouth each morning on an empty stomach, with the dose adjusted every 6 to 8 weeks based on TSH and T4 blood tests.

Unithroid

Unithroid is also a brand-name synthetic thyroid hormone (levothyroxine sodium), chemically identical to what a healthy thyroid produces, used for hypothyroidism and TSH suppression in thyroid cancer care. It's taken the same way, once daily by mouth before breakfast, with doses adjusted every 6 to 8 weeks based on blood work.

Synthroid vs. Unithroid Side Effects

Synthroid

Most common
Headache (not specified)
Nervousness (not specified)
Insomnia (not specified)
Fatigue (not specified)

Unithroid

Most common
Headache (not specified)
Nervousness (not specified)
Insomnia (not specified)
Fatigue (not specified)

Synthroid vs. Unithroid Dosage

Synthroid

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 1.6 mcg/kg/day At the start
Maximum dose Lowest dose for normal TSH If needed
Dose changes 4 increases As tolerated

Unithroid

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 1.6 mcg/kg/day At the start
Maximum dose Lowest dose for normal TSH If needed
Dose changes 4 increases As tolerated

Synthroid vs. Unithroid Indications

Synthroid

Synthroid replaces thyroid hormone in people whose thyroid gland doesn't make enough on its own, whatever the underlying cause. It's also used after thyroid cancer surgery to keep TSH low and reduce the chance of the cancer returning.

  • Primary, secondary, and tertiary hypothyroidism

  • Pituitary TSH suppression after thyroid cancer surgery

  • Not indicated for weight loss or obesity

  • Approved for adults and pediatric patients, including neonates

Unithroid

Unithroid replaces thyroid hormone across the same range of hypothyroid conditions, from thyroid gland failure to pituitary or hypothalamic causes. Like Synthroid, it's also used to suppress TSH as part of thyroid cancer follow-up care.

  • Primary, secondary, and tertiary hypothyroidism

  • TSH suppression as part of thyroid cancer management

  • Not indicated for weight loss or obesity

  • Approved for adults and pediatric patients

Synthroid vs. Unithroid Pros and Cons

Synthroid

Synthroid is the longest-established, most recognized brand of levothyroxine, with the largest real-world prescribing history of any thyroid hormone product. That familiarity drives pharmacy availability, but it doesn't mean better absorption than other levothyroxine brands.

Pros

  • Most widely prescribed levothyroxine brand in the U.S.

  • Long, well-documented real-world track record spanning decades

  • Manufacturer savings program can lower the monthly out-of-pocket cost

Cons

  • Tablets contain talc, an inactive ingredient some patients avoid

  • Received its modern FDA approval a year after Unithroid did

  • History of potency-consistency recalls before its 2002 reformulation

Unithroid

Unithroid was the first levothyroxine tablet to complete the FDA's modern full-approval process, a year ahead of Synthroid. It offers the same clinical effect as Synthroid, but with less brand recognition among pharmacies and patients.

Pros

  • First levothyroxine tablet to complete the FDA's full-approval process, in 2001

  • Tablet formulation is talc-free

  • Manufacturer delivery program can lower the monthly out-of-pocket cost

Cons

  • Less brand recognition, so pharmacies may stock it less often than Synthroid

  • Fewer decades of real-world, brand-specific outcome data than Synthroid

  • Made by a smaller manufacturer, which can affect supply consistency

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