Levothyroxine allergy: what to know

An allergy can cause hives, itching, facial or lip swelling, wheezing or breathing difficulty soon after a dose. These symptoms require a different response from effects of too much thyroid hormone, such as shakiness, sweating, a racing heartbeat or anxiety. Symptoms such as fatigue, constipation, cold intolerance or unexplained weight gain more often reflect underactive thyroid or an inadequate dose than allergy. Tablet dyes, fillers or other excipients can also be relevant, so record the product name and timing of symptoms.

If you suspect a reaction, contact the prescriber or pharmacist before taking another dose unless emergency services instruct otherwise. Do not stop thyroid replacement without a plan, because untreated hypothyroidism can cause symptoms to return. Bring the package, dose, other medicines and a description of the reaction to the appointment. The clinician may review a thyroid function test, check for another explanation, or discuss a different formulation or carefully supervised alternative.

Call emergency services immediately for trouble breathing, throat or tongue swelling, faintness, or rapidly spreading hives. Seek same-day advice for facial swelling, widespread rash, repeated vomiting or a racing heartbeat after a dose. A mild isolated symptom still deserves review if it returns after each dose or is worsening.

Does this look like allergy, a dose effect or a reaction to an inactive ingredient? Should I have a thyroid function test? Which formulation and dosing plan can keep my thyroid replacement safe?