Allergic reactions to Synthroid: signs and what to do
Allergic reactions to medication range from mild rash to life-threatening anaphylaxis. Synthroid (Levothyroxine) is generally well-tolerated at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg, but any active ingredient including Levothyroxine or its excipients can trigger an allergic response in a sensitised individual. Knowing what to watch for and how to act is part of safe use.
Recognising an allergic reaction
Mild reactions present as itching, localised rash, or mild flushing within hours of dosing. Moderate reactions add hives, swelling of lips or face, more extensive rash, or wheezing. Severe reactions — anaphylaxis — include rapid swelling of throat, difficulty breathing, drop in blood pressure, dizziness or collapse. Severe reactions are a medical emergency.
What to do
For mild reactions, stop Synthroid and contact the prescriber for guidance — antihistamines may help symptomatically. For moderate reactions, stop the medication and seek same-day medical assessment. For any sign of anaphylaxis, call emergency services immediately. According to the prescribing information for Levothyroxine, a confirmed allergy contraindicates further use of Synthroid at any 25mcg, 50mcg, 75mcg, 100mcg, 150mcg dose.
Frequently asked questions
How do I know if I'm allergic to Synthroid? ▾
Allergic reactions typically start within hours to days of first exposure and present as itching, hives, rash or, in severe cases, breathing difficulty and swelling. If symptoms started after starting Synthroid at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg and are not explained by another cause, allergy is the working hypothesis until confirmed otherwise.
Can I switch to a different brand if I'm allergic to Synthroid? ▾
A reaction to Synthroid is usually a reaction to either the active ingredient Levothyroxine or to an excipient. If the trigger is an excipient, a different brand of the same active ingredient may be tolerated; if the trigger is Levothyroxine itself, the whole Hormones and Birth Control class may need to be avoided. The prescriber decides based on the reaction type and severity.
More on Synthroid
- With alcoholSynthroid and alcohol — is it safe to drink?
- With foodShould Synthroid be taken with food?
- Side effectsSynthroid side effects: common, rare and warning signs
- Dosage guideSynthroid dosage guide: how much to take and when
- OnsetHow fast does Synthroid start working?
- DurationHow long does Synthroid last?
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.