Comparing Synthroid vs Levothyroxine:
Side Effects, Cost, & More

Written by Kathleen Morrison
Medically Reviewed by Andrea Sleeth WHNP-BC, MSCP

Key takeaways:

• Synthroid is a brand-name prescription thyroid medication; Levothyroxine is its generic counterpart. Both contain the same active ingredient, Levothyroxine sodium, and are used to treat hypothyroidism.
• The differences between Synthroid and Levothyroxine are not about which one works "better"; they're about formulation details like inactive ingredients, manufacturing consistency, and cost.
• Generic Levothyroxine is FDA-approved and typically costs less than brand-name Synthroid at the pharmacy.
• If your provider recommends switching formulations, plan to recheck your thyroid hormone levels in 6–8 weeks and keep them informed of any changes or symptoms.

You've been prescribed Levothyroxine to treat hypothyroidism, great. Then, while reading about your medication or talking with your healthcare provider, you come across another name: Synthroid.

So, what's the difference? Are they the same medication, or does the brand name offer something different?

Here's the short answer: both Synthroid and generic Levothyroxine deliver the same synthetic thyroid hormone to your body. The differences between Synthroid and Levothyroxine come down to what surrounds that hormone, cost, insurance, and more.

So, whether you're refilling an existing prescription or simply comparing your options before your next conversation with your healthcare provider, this breakdown will help you understand the differences between brand-name Synthroid and generic Levothyroxine.

Wisp is not affiliated and does not sponsor or endorse any company names, logos, or brands shown or mentioned. All product names, logos, brands, and other trademarks or images shown or mentioned are the property of the respective brand owners and are only used to identify the products and services of their respective owners.

What are Synthroid and Levothyroxine?

Levothyroxine is used to treat hypothyroidism, also known as low thyroid function, by replacing the thyroxine (T4) that your thyroid gland isn't producing on its own.

Synthroid is a brand-name version of Levothyroxine sodium, prescribed for over 70 years to help maintain thyroid hormone levels in balance. It is manufactured by AbbVie and comes in 12 color-coded dosage strengths ranging from 25 mcg to 300 mcg.

Back to Levothyroxine, it is a generic version of the same thyroid medication, available from multiple manufacturers.

You may also see it sold under other brand names like Levoxyl, Euthyrox, Levothroid, and Tirosint, each a slightly different form of Levothyroxine with distinct formulations.

Active ingredient: Are they actually the same?

Spoiler: yes. Both contain the same active ingredient, Levothyroxine sodium, a synthetic form of thyroxine (T4), the main hormone your thyroid gland produces.

Once in your body, T4 converts to T3, which regulates your metabolism, energy, mood, and more. Whether you're prescribed Synthroid or a generic, your thyroid is getting the same hormone replacement.

That's because the Food and Drug Administration (FDA) requires every generic drug to contain the same active ingredient as its brand-name counterpart, and generic Levothyroxine is no exception.

Inactive ingredients: Where the real differences live

This is where things get more nuanced, and where some people actually feel a difference.

Inactive ingredients are the fillers, binders, and dyes that give a pill its shape and color. They don't change what the thyroid hormone does once it's in your bloodstream, but inactive ingredients may affect how well your body absorbs the medication and how well you tolerate it.

Synthroid

• Contains a specific, long-standing set of inactive ingredients that has remained largely consistent
• Color-coded dosage strengths may contain dyes that cause reactions in some sensitive individuals
• Per the official prescribing information, foods like soybean flour, cottonseed meal, walnuts, and dietary fiber may reduce Synthroid absorption; grapefruit juice may also reduce its effect
  

Generic Levothyroxine

• Inactive ingredients vary by manufacturer; switching between one generic manufacturer and another is not the same as staying on the same product
• Some generic products contain lactose, which may be an issue for those with lactose intolerance
• The FDA has approved many generic Levothyroxine products, and pharmacists in most states may substitute them for Synthroid unless a provider specifies otherwise. That said, because Levothyroxine has a narrow therapeutic index, the American Thyroid Association and other endocrine societies advise against switching formulations or manufacturers without your provider's knowledge, since even small differences in absorption can shift thyroid hormone levels.

If you're opting for generic Levothyroxine, a quick word with your pharmacist about inactive ingredients or maintaining the same generic manufacturer (that previously worked) for each refill can save a lot of guesswork.

Cost and insurance: The part your wallet cares about

Let's be honest...for many people with hypothyroidism, managing a lifelong condition, cost is not a small detail. It's often the deciding factor.

Synthroid

• The average retail price for Synthroid oral tablets runs approximately $160–$161 for a 90-tablet supply across most dosage strengths, without insurance or discount programs
• AbbVie offers a Synthroid copay savings card for commercially insured patients and a Synthroid Delivers Program with home delivery pricing starting at around $25/month for a 90-day prescription. But eligibility restrictions apply, and these programs are not available to patients on federal or government-funded insurance programs
• Most insurance plans cover Synthroid, but it typically falls on a higher copay tier than generic options
  

Generic Levothyroxine

• Generic Levothyroxine oral tablets are considerably less expensive than brand-name Synthroid at most pharmacies, often available for a fraction of the cost without insurance
• Most insurance plans cover generic Levothyroxine at the lowest copay tier, making it the most cost-accessible option for people with hypothyroidism
• For those without an insurance plan, generic Levothyroxine is widely available at major pharmacies at low out-of-pocket prices

If staying consistent with your thyroid medication is ever a barrier because of cost, generic Levothyroxine vs Synthroid becomes a very practical conversation to have with your provider.

For most people, affordability and adherence go hand in hand, and that matters for long-term thyroid function.

Bioavailability and dosing: A precision sport

You might think: if generic Levothyroxine costs less, does that mean it's somehow less precise or less appropriate than brand-name Synthroid? Not quite.

The FDA requires generic and brand-name medications to fall within an accepted bioavailability range compared to the reference brand-name drug.

For most people with hypothyroidism, this variation doesn't cause any noticeable difference.

The treatment of hypothyroidism requires precision, but that comes from taking the same formulation consistently, not from choosing a brand over a generic. Whether it's Synthroid or a specific generic, the key is to stick with the same one.

A few things to know about taking Synthroid or Levothyroxine:

• Take it on an empty stomach, 30–60 minutes before eating, at the same time every day.
• Calcium, iron, and antacids can interfere with absorption; space them out.
• Recheck thyroid levels 6–8 weeks after any dosage change or formulation switch.
  

Side effects of Synthroid and Levothyroxine

Because they share the same active ingredient, the side effects of Synthroid and Levothyroxine are essentially the same. Most side effects occur when your dosage is too high or too low, not because of the medication itself.

Side effects of high dose

• Heart palpitations or increased heart rate
• Tremors (shakiness)
• Increased sweating or heat intolerance
• Unintended weight loss
• Diarrhea or increased bowel movements
• Menstrual irregularities
  

Side effects of low dose

• Persistent fatigue or sluggishness
• Cold intolerance
• Constipation
• Dry skin or hair loss
  

Note: In addition to potential side effects, there are important precautions to keep in mind before taking Synthroid or levothyroxine: You should not take Synthroid or levothyroxine if you are allergic to any of their ingredients, including glycerin or edetate disodium, or if you have an untreated or uncontrolled adrenal gland disorder. Certain foods, supplements (such as calcium or iron), and medications can interfere with how well levothyroxine is absorbed. Before starting treatment, tell your healthcare provider about all medications, supplements, and medical conditions to help ensure safe and effective use.

This is not a complete list of risks and side effects, and others may occur. If symptoms worsen or you notice signs of an allergic reaction, contact a provider right away.

When to switch

Switching from brand to generic (or vice versa) isn't always a big deal. But there are times when it's worth being intentional about it.

Reasons you might switch from Synthroid to generic:

• Cost is a barrier to consistent use
• Your thyroid levels have been stable for a while
• Your provider and pharmacist confirm the switch is appropriate
  

Getting your thyroid medication refilled, without the wait

If your thyroid medication is running low and you can’t get to your provider in time, Wisp’s Thyroid Medication Refill was built for moments like this.

For patients with an existing hypothyroidism diagnosis and prescription, Wisp helps bridge the gap with a 90-day supply of thyroid medication (generic Levothyroxine or Synthroid).

Start your consult and help avoid an unnecessary gap in treatment.

This blog post is for informational and educational purposes only and should not be taken as professional advice. Always consult with a qualified professional before making any decisions based on the information provided here.