Allergic Reactions and Sensitivities Caused by Synthroid

This occurs more commonly in some than others

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Synthroid can make people who are living with hypothyroidism feel much better, but allergic reactions or sensitivities to the drug may occur due to ingredients such as acacia, lactose, and cornstarch.

Symptoms ranging from a runny nose to a rash and/or hives, to abdominal pain may occur, and it may take some time to realize it's the drug rather than something else that's responsible. Hypersensitivity reactions to Synthroid are more common among those who have allergies, hay fever, or asthma.

An allergic reaction to Synthroid is most often diagnosed based on history, though allergy tests are available for acacia. Management most often includes switching to another brand of levothyroxine, and, fortunately, there are other brands of the drug with different ingredients that may be effective.

Synthroid package
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Allergens in Synthroid

In addition to the active ingredient, in this case, thyroid hormone, most medications also contain inactive ingredients, known as excipients. While these ingredients are inactive, they are not necessarily inert and may give rise to allergic reactions or other symptoms. Many allergic reactions and sensitivities to Synthroid are related to acacia, lactose, or cornstarch, though very rarely, allergic reactions to levothyroxine itself have been noted.


Acacia is a family of shrubs and trees, and it is used as an ingredient (acacia gum) in some medications, including Synthroid brand levothyroxine, to provide form and shape to tablets. Some people who have pollen allergies and hay fever—especially to tree and grass pollens (such as ryegrass pollen)—may also have an allergy to acacia, even when it's an ingredient in a medication.

People who have asthma are also more likely to be allergic. For some people with hypothyroidism who have these allergies, taking Synthroid can cause allergic symptoms.

Interestingly, it also appears that people who have seasonal allergies may find that they don't respond well to their Synthroid during allergy season.

Studies looking at the incidence of acacia sensitivity are few, but it appears that there is a particularly high rate of sensitization among people living in Iran and surrounding countries, as well as the Philippines.


Another ingredient in Synthroid is lactose, which can trigger symptoms in people with lactose intolerance. Lactose intolerance is an inability to digest lactose, the major sugar found in milk. Lactose is also an ingredient in some foods and medications. Signs of lactose intolerance may include abdominal pain, cramps, bloating, gas, nausea, and diarrhea.

With lactose intolerance, these symptoms often begin 30 minutes to two hours after taking Synthroid.


In addition to acacia and lactose, one of the more common fillers used in Synthroid is confectioner’s sugar (powdered sugar), which contains cornstarch. Some studies have found that corn proteins cross-react with gluten, which could trigger an immune system reaction to the fillers in the same way that it does to gluten. Cornstarch may also be a problem for those who have a corn allergy.

While this cross-reactivity may occur and affect people with celiac disease or non-celiac gluten sensitivity, gluten itself does not appear to be a problem. A 2017 study looked specifically at the gluten content of Synthroid tablets.

The level of gluten in Synthroid was found to be below detectable levels (based on FDA criteria, Synthroid would be considered gluten-free), and researchers felt that, even though the threshold of gluten necessary to cause worsening of celiac disease is unknown, it's unlikely that Synthroid would exacerbate symptoms in people living with celiac disease.

Signs and Symptoms

The signs and symptoms of an allergy or hypersensitivity to Synthroid can take several forms.

With acacia allergy, symptoms often include a runny nose, runny eyes, and congestion, though mood changes may occur as well. Some people develop an eczematous-type rash, hives, or widespread itching.

With lactose intolerance, the most common symptoms are abdominal discomfort, bloating, gas, nausea, and vomiting.

Signs of a cornstarch allergy can depend on the underlying sensitivity. With a corn allergy, symptoms may range from hives to hay fever, to anaphylaxis. With celiac disease, symptoms may suggest an intolerance, such as bloating, abdominal pain, or constipation, but may also include less common symptoms ranging from anemia to infertility.

Some people have recognized an allergy by noting that their Synthroid doesn't seem to work as well at certain times of the year, for example, during hay fever season. If your TSH fluctuates from high to low, there are many potential causes, but hypersensitivity to Synthroid is one possibility.

As an example, a person might find that their TSH numbers bounce around, with high TSH spikes during allergy season and low TSH levels during the winter months.

While allergic reactions to Synthroid are uncommon, any allergic reaction has the potential to be life-threatening. You should seek immediate medical attention if you develop lightheadedness, shortness of breath, wheezing, chest pain, or other signs of anaphylaxis (a severe allergic reaction).


The diagnosis of an allergic reaction or sensitivity to Synthroid can be challenging, as many of the people who have these allergies also have hay fever, lactose intolerance, or gluten sensitivity. Most often, the diagnosis is made by taking a careful history of symptoms, including how they relate to the timing of Synthroid dosing, meals, and exposure to other allergens.

If you are concerned, keeping a daily diary of symptoms may be helpful. Make sure to document any hay fever symptoms, hives or itching, and digestive system symptoms you have, as well as how you are feeling overall. An allergy may result in TSH levels that rise (and consequently, symptoms of hypothyroidism), but there are many possible causes if your thyroid medication isn't working.

Allergy testing is available for acacia, though the allergy may be suspected in those who have previously been diagnosed with tree or grass allergies.

There are several tests that can be done if lactose intolerance is suspected, such as a lactose tolerance test, hydrogen breath test, or, in children, a stool acidity test. That said, many people learn of the intolerance themselves, and an elimination diet is often the best way to diagnose the condition along with any other food intolerances.

The same is true for corn allergy, for which skin and blood tests are available, but often inaccurate.

If Synthroid could be a gluten trigger, perhaps the easiest way to determine this is to try a different brand of the drug (under a healthcare provider's direction).

Management and Treatment

If you suspect you are sensitive to acacia, lactose, corn, or possibly a gluten trigger in your Synthroid, it's important to raise this to your healthcare provider. What she will recommend depends, in large part, on the severity of the reaction you are having.

For Mild Symptoms or Uncertain Allergies

If your symptoms are mild, you may wish to continue the medication and keep a symptom diary, using allergy medications as needed to control your symptoms until you have a better idea if you have a problem with Synthroid or not.

With lactose intolerance, there is the option of using lactase supplements (which contain the enzyme that is needed to break down lactose) as well, if you do not wish to change your medication. Lactase supplements themselves, however, may trigger allergic reactions in some people.

Switching Thyroxine Brands

While, in general, you should stay on the same brand of levothyroxine to best treat your hypothyroidism, switching to a different brand may not only alleviate your symptoms, but confirm a possible allergy. Before switching medications, talk with your healthcare provider and make sure that you need to continue with thyroid replacement (most people will, but not all).

Levoxyl and Tirosint are brands of levothyroxine that are free of both acacia and lactose. Tirosint seems to be particularly effective for treating people who have celiac disease in addition to hypothyroidism, as it has fewer active ingredients. It may, however, cause heart problems, as well as difficulty controlling blood sugar in people who have diabetes.

There are also other brands of levothyroxine such as Levothroid and Unithroid. There are also many brands of generic levothyroxine, though there has been some controversy over the equivalence of these products.


Oral desensitization, or immunotherapy for allergies (creating a tolerance to the medication as with allergy shots), is not commonly used when a Synthroid allergy is diagnosed but has been effective for some people who appear to have a true allergy to levothyroxine.

A Word From Verywell

Allergies or sensitivities to Synthroid are not common, but can and do occur. Fortunately, there are other brands of levothyroxine available if switching your medication is considered. Listening to your body, and being your own advocate in your care, is critical both in making the proper diagnosis and in choosing the best option for moving forward.

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9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Mary Shomon
Mary Shomon is a writer and hormonal health and thyroid advocate. She is the author of "The Thyroid Diet Revolution."