What ingredients in generic thyroid medication, specifically levothyroxine (T4) and liothyronine (T3), can cause an allergic reaction?

From the FDA Drug Label

Levothyroxine sodium tablets, USP contain no ingredients made from a gluten-containing grain (wheat, barley, or rye). Each levothyroxine sodium tablet contains the inactive ingredients microcrystalline sodium, light magnesium oxide, sodium starch glycolate and sodium stearyl fumarate. Table 9 provides a listing of the color additives by tablet strength: Table 9 Levothyroxine Sodium Tablet Color Additives Strength (mcg) Color additive(s) 25 FD&C Yellow No. 6 Aluminum Lake 50 None 75 FD&C Red No. 40 Aluminum Lake, FD&C Blue No. 2 Aluminum Lake 88 FD&C Blue No. 2 Aluminum Lake, FD&C Yellow No. 5 Aluminum Lake 100 FD&C Yellow No. 5 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lake 112 FD&C Red No. 40 Aluminum Lake, Carmine 125 FD&C Yellow No 6 Aluminum Lake, FD&C Red No. 40 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake 137 FD&C Blue No. 1 Aluminum Lake 150 FD&C Blue No. 2 Aluminum Lake 175 FD&C Blue No. 1 Aluminum Lake, Carmine 200 FD&C Red No. 40 Aluminum Lake 300 D&C Yellow No. 10 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake

The ingredients in generic thyroid medicine that may cause an allergy are:

• Color additives:
  • FD&C Yellow No. 6 Aluminum Lake
  • FD&C Red No. 40 Aluminum Lake
  • FD&C Blue No. 2 Aluminum Lake
  • FD&C Yellow No. 5 Aluminum Lake
  • Carmine
  • D&C Yellow No. 10 Aluminum Lake
  • FD&C Blue No. 1 Aluminum Lake
• Inactive ingredients:
  • Microcrystalline sodium
  • Light magnesium oxide
  • Sodium starch glycolate
  • Sodium stearyl fumarate 1 1 1

From the Research

Generic thyroid medications can cause allergic reactions due to certain inactive ingredients they contain, including lactose, dyes like FD&C Yellow No. 5 (tartrazine), gluten, acacia, corn starch, povidone (PVP), sodium starch glycolate, magnesium stearate, and various preservatives. According to a study published in 2020 2, the effectiveness of generic vs brand-name levothyroxine in achieving normal thyrotropin levels was compared, and the results suggest that generic levothyroxine is as effective as brand-name levothyroxine. However, this study does not directly address the issue of allergic reactions to inactive ingredients in generic thyroid medications.

Some key points to consider when evaluating the potential for allergic reactions to generic thyroid medications include:

• Lactose intolerance: Some generic thyroid medications may contain lactose, which can cause reactions in individuals with lactose intolerance.
• Dyes and preservatives: Certain dyes, such as FD&C Yellow No. 5 (tartrazine), and preservatives may cause reactions in sensitive individuals.
• Gluten: Generic thyroid medications may contain gluten, which can be problematic for individuals with celiac disease.
• Other potential allergens: Corn starch, povidone (PVP), sodium starch glycolate, magnesium stearate, and acacia are other potential allergens that may be present in generic thyroid medications.

It is essential to note that the active ingredient levothyroxine itself rarely causes allergic reactions, as stated in a study published in 2019 3. If an allergy to a thyroid medication is suspected, it is crucial to consult a healthcare provider immediately, as they can help determine the specific allergen and switch to a different formulation or brand that does not contain the problematic ingredient. Different manufacturers use different inactive ingredients, so changing brands might resolve the issue while maintaining proper thyroid hormone replacement.