Synthroid: a generic Levothyroxine that may behave differently

Reviewed by Chimene Richa, MD, 

Written by Andrea Pierce 

Reviewed by Chimene Richa, MD, 

Written by Andrea Pierce 

last updated: Oct 12, 2020

5 min read

Is Synthroid better than generic levothyroxine? The first person to ask is your healthcare provider. Knowing you and any medical conditions you have, they may have a good reason for recommending one over the other. Ask them to explain! Cost may be a factor, too. 

For the most part, studies don’t show clear advantages of one type—brand name or generic—over others. But people’s experiences tell another story, with some describing feeling much better after changing from one formulation to another. 


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Levothyroxine uses

Hypothyroidism is a medical condition where your thyroid gland does not produce enough thyroid hormones to keep your body’s functions in balance. 

If you’ve been diagnosed with hypothyroidism, whether from an autoimmune thyroid disease like Hashimoto’s disease or thyroiditis, surgery, radiation, or just a low thyroid function, you will need thyroid hormone replacement therapy. 

Most people are prescribed levothyroxine, a man-made (synthetic) form of thyroxine (T4), the primary hormone that your thyroid gland naturally produces. Thyroxine plays an essential role in regulating your body’s metabolism, temperature, digestion, and many other processes—this is why it is so important to replace it (NIDDK, 2016). Thyroid hormones are also used to treat some forms of thyroid cancer.

Synthroid uses

Synthroid is brand name levothyroxine sodium; other brand name versions of levothyroxine include Levothroid, Unithroid, Tirosint, and Levoxyl. It contains the same active ingredients and is used for the same purposes, namely to treat hypothyroidism or low thyroid hormone levels.

Levothyroxine vs. Synthroid: which is better?

Many people wonder if levothyroxine (or levothyroxine sodium) and Synthroid are the same—and, if not, which might be better. With most medicines, it doesn’t matter if you take the generic or brand name version or if you switch between them.

The FDA makes sure that both have the same active ingredients, work the same way, and have the same side effects. Then there is the cost to consider—generic medicines are usually cheaper. 

However, this may not be the case with brand name and generic thyroid replacement medicine, like levothyroxine and Synthroid. Endocrine and thyroid organizations like the American Thyroid Association, the American Association of Clinical Endocrinologists, and The Endocrine Society are wary that the generic and branded levothyroxine may not behave the same way for every person (Benvenga, 2019). 

This isn’t confirmed, but the three organizations have put out a statement saying they are concerned about the FDA’s method for determining bioequivalence (similarity between the drugs).

While the active ingredient (levothyroxine sodium) may be the same, other “inactive ingredients” typically added to drug formulations differ between the generic and brand name medications. The inactive ingredients are necessary to help preserve the drug, absorb the medicine, etc. 

Some of these inactive ingredients may affect how fast you absorb the medication and potentially how you respond to the hormone. Even small differences could mean that you have a significant difference in the levels of thyroid hormone delivered throughout your body from brand to brand or brand to generic formulations—this will ultimately affect how you feel (Benvenga, 2019).

In general, if you buy the same brand name hormone or generic drug from the same manufacturer, batches are relatively consistent in terms of these inactive ingredients, since the same manufacturer makes them. However, be aware that different manufacturers can make the generic forms of levothyroxine—check with your pharmacy to see which generic manufacturer they use and keep it consistent. 

In the United States, depending on your insurance plan, your pharmacist may be required to switch your brand name thyroid medication to generic, unless specified by your healthcare provider. 

Overall, neither the brand name nor generic is necessarily better than the other. However, once you find which one works best for you, stay with the same brand or manufacturer. If you change brands, switch to generic, or switch from one generic to another, you may need to get thyroid stimulating hormone (TSH) blood tests checked after six weeks to ensure that your new hormone therapy is effective. Then, follow up with your provider. You may need a dose adjustment if your TSH levels are suboptimal (Benvenga, 2019).

Side effects of Synthroid and levothyroxine

Because the active chemicals in both brand name and generic levothyroxine are the same, they share the same side effect profile. Most of the side effects of both medications come from getting too much of the synthetic hormone, leading to hyperthyroid (too much thyroid hormone) symptoms.

The U.S. Food and Drug Administration (FDA) has issued a black box warning regarding thyroid replacement hormones: Do not use thyroid hormones, like levothyroxine or Synthroid, for weight loss or to treat obesity. Large doses can lead to serious and life-threatening effects (DailyMed, 2019).

Common side effects include (DailyMed, 2019):

  • Excessive sweating (hyperhidrosis)

  • Nervousness/anxiety

  • Tiredness

  • Muscle tremors

  • Diarrhea

  • Hair loss

  • Increased appetite

  • Weight loss

  • Inability to tolerate high temperatures (heat intolerance)

  • Fever

  • Decreased bone mineral density

  • Hyperactivity

  • Irregular menstrual cycles

  • Fertility problems

If your thyroid hormone replacement dose is too high, you may experience more severe side effects, including (DailyMed, 2019): 

  • Fast heartbeat (tachycardia or palpitations)

  • Irregular heartbeat (arrhythmia)

  • High blood pressure

  • Heart failure

  • Chest pain (angina)

  • Heart attack (myocardial infarction)

  • Cardiac arrest (heart stops working)

Synthroid and levothyroxine dosage

The dosing for both brand name and generic forms of thyroid hormone replacement is the same. Overall, the goal is to effectively replace your body’s thyroid hormone to get your levels back to normal.

Most people use the tablet or capsule form of the medications. Generic and brand name pills are available in a variety of doses, including 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg (UpToDate, n.d). You should take thyroid hormones on an empty stomach, at least 30–60 minutes before eating any food. Certain antacids, like those with calcium carbonate or proton-pump inhibitors PPIs), can interfere with hormone absorption, so avoid taking levothyroxine or Synthroid with these drugs (DailyMed, 2019). Seek medical advice from your healthcare provider or pharmacist regarding other possible drug interactions. 

After starting Synthroid or levothyroxine, your healthcare provider will likely check your thyroid-stimulating hormone (TSH) blood tests to see if you are getting enough hormone replacement. A high TSH level means that you are not getting enough thyroxine and vice versa. If your dose changes or you change manufacturers, the TSH will need to be repeated. 

Cost and coverage for Synthroid and generic levothyroxine

Most insurance plans cover thyroid hormone replacement. However, you should check your coverage plan carefully and speak with your pharmacist before starting on a specific formulation to make sure. The cost of a 30-day supply ranges from $4 to over $50, depending on the strength and whether it is a brand name or generic (, n.d.). 


The bottom line: The American Thyroid Association recommends that when you find a formulation that works for you, you stick to the same brand name or generic medication (American Thyroid Association, n.d.). If you need to change, you should let your healthcare provider know as you may need to repeat thyroid testing to make sure that the drug works for you.  

For an illness like hypothyroidism that typically requires lifelong therapy, consistent and precise treatment with the same thyroid hormone brand over time maximizes your chances of effective treatment (American Thyroid Association, n.d.).  

Once you have chosen your brand or generic formulation, you can ask your healthcare provider to write “DAW,” which means “dispense as written,” or “no generic substitution” on your subscription to prevent medication changes.


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

October 12, 2020

Written by

Andrea Pierce

Fact checked by

Chimene Richa, MD

About the medical reviewer

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.