Synthroid or levothyroxine can make people living with hypothyroidism feel much better, however allergic reactions or drug sensitivities can occur due to ingredients such as acacia, lactose, and cornstarch . 

Symptoms ranging from a runny nose to skin rash and / or hives to abdominal pain can occur, and it may take a while to realize that the medication is to blame rather than something else. Hypersensitivity reactions to Synthroid are more common among those with allergies, hay fever, or asthma.

An allergic reaction to Synthroid is most often diagnosed based on history, although allergy tests are available for acacia. Managing this problem 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.

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 can lead to allergic reactions or other symptoms. 

Many allergic reactions and sensitivities to Synthroid are related to acacia, lactose, or cornstarch, although allergic reactions to levothyroxine have been observed very rarely.

1. Acacia

Acacia is a family of shrubs and trees, and is used as an ingredient (acacia gum) in some medications, including Synthroid brand levothyroxine, to give tablets shape and structure. 

Some people who have allergies to pollen and hay fever, especially to pollens from trees and herbs (such as rye grass pollen), may also have an allergy to acacia, even when it is an ingredient in a medicine. 

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

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

Studies looking at the incidence of acacia sensitivity are few, but there appears to be a particularly high rate of awareness among people living in Iran and neighboring countries, as well as the Philippines.

2. Lactose

Another ingredient in Synthroid is lactose, which can trigger symptoms in people with lactose intolerance. Lactose intolerance is an inability to digest lactose, the main sugar found in milk. 

Lactose is also an ingredient in some foods and medicines. The signs of lactose intolerance can include abdominal pain, cramping, bloating, gas, nausea and diarrhea.

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

3. Corn starch

In addition to acacia and lactose, one of the most common fillers used in Synthroid is confectionery 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 fillers in the same way that it does to gluten. Cornstarch can also be a problem for those with a corn allergy.

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

The level of gluten in the Synthroid was found to be below detectable levels (based on FDA criteria, the Synthroid would be considered gluten-free). 

And the researchers felt that while the gluten threshold required to cause worsening celiac disease is unknown, Synthroid is unlikely to exacerbate symptoms in people with celiac disease.

Signs and Symptoms of Allergic Reactions to Synthroid

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

In the case of acacia allergy, symptoms often include a runny nose, eye discharge, and congestion, although mood swings can also occur. Some people develop an eczematous-like rash, hives, or generalized itching.

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

The signs of a cornstarch allergy may depend on the underlying sensitivity. When there is an allergy to corn, symptoms can range from hives to hay fever to anaphylaxis . 

For celiac disease, symptoms can suggest an intolerance, such as bloating, abdominal pain, or constipation , but can also include less common symptoms ranging from anemia to infertility.

Some people have recognized that they have 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, and hypersensitivity to Synthroid may be one of them.

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

Diagnosis of this type of allergies

Diagnosing 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 are related to the timing of Synthroid dosing, foods, and exposure to other allergens.

If you are concerned about having an allergy to Synthroid, it may help to keep a diary of your symptoms. Be sure to document any symptoms of hay fever, hives, or itching and digestive system symptoms you have, as well as how you feel in general. 

An allergy can cause TSH levels to rise (and consequently hypothyroid symptoms), but there are many possible causes if your thyroid medication doesn’t work.

There are allergy tests available for acacia, although the allergy may be suspected in those who have previously been diagnosed with allergies to trees or herbs.

There are several tests you can take if you suspect lactose intolerance, such as a lactose tolerance test, a hydrogen breath test, or, in children, a stool acidity test. 

That said, many people learn of their intolerance on their own, and often the best way to diagnose the condition in conjunction with any other food intolerance is to follow an elimination diet.

The same applies in the case of allergy to corn, for which skin and blood tests are available, but often inaccurate.

If the Synthroid you ingest is a gluten trigger, perhaps the easiest way to determine it is to try a different brand of levothyroxine (under the direction of a physician).

Management and treatment of allergies to Synthroid

If you suspect that you are sensitive to acacia, lactose, corn, or possibly a gluten trigger in your Synthroid, it is important that you report this to your healthcare provider. What he will recommend depends, in large part, on the severity of the reaction you are experiencing.

1. For mild symptoms or uncertain allergies

If your symptoms are mild, you may want to continue medication and keep a symptom diary, using allergy medications as needed to manage your symptoms until you have a better idea of ​​whether you really have a problem with Synthroid or not.

In case of lactose intolerance, there is also the option of using lactase supplements (which contain the enzyme that is needed to break down lactose), if you do not want to change your medication. However, lactase supplements themselves can trigger allergic reactions in some people.

2. Change brands of thyroxine

While you should generally stick with the same brand of levothyroxine to better treat your hypothyroidism, switching to a different brand can not only ease your symptoms, but confirm a possible allergy. 

Before changing medications, talk to your doctor and make sure you need to continue thyroid replacement (most people will, but not all).

Levoxyl and Tirosint are brands of levothyroxine that are acacia and lactose free. Tirosint appears to be particularly effective in treating people who have celiac disease in addition to hypothyroidism, as it has fewer active ingredients. However, it can cause heart problems, as well as difficulties controlling blood sugar in people with diabetes.

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

3. Desensitization

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

By Dr. Eric Jackson

Dr. Eric Jackson provides primary Internal Medicine care for men and women and treats patients with bone and mineral diseases, diabetes, heart conditions, and other chronic illnesses.He is a Washington University Bone Health Program physician and is a certified Bone Densitometrist. Dr. Avery is consistently recognized in "The Best Doctors in America" list.

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