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You are taking levothyroxine tablets -- Levoxyl®, Synthroid®, Unithroid®, or generic levothyroxine -- and you still don't feel well...and you may also be lactose intolerant, have seasonal allergies/hay fever, gluten sensitivity, or you’re sensitive to iodine.


You may be sensitive or allergic to the dyes or fillers and binders – known as excipients -- found in most levothyroxine tablets.

Every dosage of levothyroxine in tablet form – except the 50 mcg white tablets – contains dyes, that give the tablets their color. While the dyes are FDA-approved, they have been banned by other countries, and various experts and advocates, including the Center for Science in the Public Interest, a consumer watchdog group, have petitioned the FDA to ban the use of dyes due to potential health risks. The dyes included in levothyroxine tablets include:

  • FD&C Blue No. 1 Aluminum Lake: Can cause hypersensitivity reactions. In 2003, the FDA issued a Public Health Advisory to alert healthcare providers of toxicity associated with some uses.

  • FD&C Blue No. 2 Aluminum Lake: The Center for Science in the Public Interest says that it is not safe for human consumption. The Environmental Working Group cites numerous studies looking at the potential toxicity.

  • FD&C Red No. 27 Aluminum Lake: Classified as expected to be toxic or harmful in Canada.

  • FD&C Red No. 30 Aluminum Lake: Classified as expected to be toxic or harmful in Canada.

  • FD&C Red No. 40 Aluminum Lake: Can cause hypersensitivity reactions in some people.

  • FD&C Yellow No. 10 Aluminum Lake: Limited evidence of any toxicity.

  • FD&C Yellow No. 6 Aluminum Lake: Can cause hypersensitivity reactions in some people.


Levothyroxine tablets also have a long list of excipients – extra ingredients beyond the levothyroxine. If you are sensitive or allergic to these filler ingredients in your levothyroxine tablets, it can interfere with the medication’s effectiveness. ingredients that are most commonly associated with sensitivity, especially:

  • Acacia – a derivative of tree bark known to cause sensitivity and reactions in people with seasonal and pollen allergies/hay fever

  • Crospovidone/Povidone – which is problematic for people with iodine-sensitivity

  • Lactose – which can cause reactions in people with lactose intolerance

Note: some starch glycolate contains gluten, and can cause issues for some people sensitive to gluten

The following chart shows all the key excipient ingredients in tablets.  The most problematic excipients are highlighted in orange. They key is to avoid any of the excipients in tablets that are problemtic for you.

You can also review detailed lists of excipients for all brands of levothyroxine:

You can also learn more about the different excipients, and how they affect you, in Part 1 of the Levothyroxine Deep Dive.


If you suspect or know that you’re sensitive to dyes in levothyroxine tablets, you have two options:

If you are sensitive to excipients and other ingredients in  levothyroxine tablets, you have two options:

  • Switch to a levothyroxine tablet that does not include the problematic ingredient(s)

  • Switch to Tirosint® capsules or Tirosint®-SOL oral solution. Tirosint® capsules have three excipients: gelatin, glycerin, and water. Sensitivity or allergies to gelatin and glycerin are extremely rare. Tirosint®-SOL oral solution has two excipients: glycerol and water. Sensitivity or allergies to glycerol are also extremely rare.


The Levothyroxine Deep Dive program is copyright © 2020, Mary Shomon. All rights reserved.

Levoxyl®, Synthroid®, Tirosint®, Tirosint-SOL®, and Unithroid® are registered trademarks. Product images and logos used with permission. 


Mary Shomon does not provide medical advice, diagnosis or treatment.


The contents of this video and material contained on the website ("content") are for informational purposes only. The content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have learned from this video or site.

If you think you may have a medical emergency, call your doctor or 911 immediately. Mary Shomon does not recommend or endorse any specific tests, physicians, products, procedures, opinions, or other information that may be mentioned. Reliance on any information provided by Mary Shomon is solely at your own risk.

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