Drug & supplement interaction

Thyroid Extract + Pseudoephedrine, Terfenadine

Pseudoephedrine, Terfenadine brand name: Teldafen.

Minor Interaction
Medically reviewed by · Updated November 2023
The bottom line Minor

Thyroid Extract has a minor interaction with Pseudoephedrine, Terfenadine. Food and medications can interfere with thyroid hormone absorption.

Read the full interaction report
SeverityMinor
LikelihoodLikely
EvidenceA
What do Severity, Likelihood & Evidence mean?
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Interaction report

Thyroid Extract and Pseudoephedrine, Terfenadine

Here is the interaction we found between these two, with the evidence behind it.

Minor

Oral Drugs

Why this applies to Pseudoephedrine, Terfenadine

Pseudoephedrine, Terfenadine falls under Oral Drugs — the group Thyroid Extract interacts with.

Food and medications can interfere with thyroid hormone absorption. Some foods, particularly those high in fiber or containing certain minerals like calcium or iron, can bind to thyroid hormones and reduce their absorption in the digestive tract. Additionally, taking thyroid medications with certain drugs, like antacids or calcium supplements, can hinder the effectiveness of the thyroid medication, necessitating adjustments in dosage or timing. Prescription thyroid medications, such as Synthroid, are recommended to be taken on an empty stomach, at least 30 minutes before food and other medications, for optimal absorption.
Severity Minor Likelihood Likely Evidence A

Well-controlled human studies have demonstrated the likely existence of this interaction.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here the concern flows the other way from what you might expect. Your combination cold-and-allergy product (pseudoephedrine plus terfenadine) isn't being changed by the thyroid extract. Instead, taking them together can get in the way of how well the thyroid hormones in the extract are absorbed. When you swallow pills alongside food or other medicines, some of that thyroid hormone can bind up in your gut and not get into your body fully.

This is a minor, very manageable timing issue, not a dangerous reaction. The simple fix most pharmacists suggest is spacing them out, since thyroid hormone works best on an empty stomach, taken well before other medicines. Please run your exact schedule by your pharmacist or doctor before changing anything.

Contact Dr. Brian directly if you have additional questions on this interaction

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

Thyroid Extract Also Interacts With Other Medications

Thyroid Extract has 3 moderate, 2,018 minor known drug interactions in our database — the Pseudoephedrine, Terfenadine one above is one of them. If you take anything else, check it against the full list below.

Moderate 3 Minor 2,018 No known interaction 809 (incl. Pseudoephedrine, Terfenadine)
Taking anything else?

Check Another of Your Medications Against Thyroid Extract

Covers the 2,830 medications we’ve checked against Thyroid Extract. Pick one to open its full report.

Everything else you take

See Every Supplement That Interacts With Pseudoephedrine, Terfenadine

This page covers Thyroid Extract only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Pseudoephedrine, Terfenadine page lists 105 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), American Hellebore (major), Arsenic (major), Cesium (major) and Ephedra (major).

By severity: 10 major · 77 moderate · 18 minor.

From our pharmacist’s overview on the Pseudoephedrine, Terfenadine page

“Teldafen pairs the decongestant pseudoephedrine with terfenadine, an older antihistamine known for disturbing heart rhythm, so I read this list with real seriousness and you should too. The riskiest entries either stretch the heart's QT interval further, which is how dangerous arrhythmias begin (grapefruit in large amounts is one), or…”

Reviewed by 2 · Jul 18, 2026

Open the Pseudoephedrine, Terfenadine interaction page

How much people look this up

Where Thyroid Extract and Pseudoephedrine, Terfenadine Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Thyroid Extract
Top half

#601 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Pseudoephedrine, Terfenadine
Top 25%

#675 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Supplement

About Thyroid Extract

What is Thyroid Extract?

Thyroid Extract
  • Hypothyroidism (underactive thyroid)
  • Fatigue and low energy
  • Weight management
  • Sluggish metabolism
  • General thyroid support

Thyroid extract supplements are made from dried animal thyroid gland and may contain real thyroid hormones (T3 and T4), but the amounts in over-the-counter products are not standardized and can be unpredictable. Self-treating a thyroid problem with these supplements can be risky, so always work with a doctor or pharmacist and use prescription thyroid medicine when treatment is needed.

Read the full Thyroid Extract monograph & all interactions
Keep checking

Other Drugs & Thyroid Extract

Browse every other medication we’ve checked against Thyroid Extract — both the ones with a known interaction and the ones without. Select any drug for the full detail.

0Major
3Moderate
2,018Minor
808No known interaction
3 Moderate 2,018 Minor 808 No interaction
Moderate

3 interactions

Minor

2,018 interactions

Showing 60 of 2,018 minor interactions — see all 2,018 on the Thyroid Extract page.

No interaction 808 drugs checked — show the list

We also checked Thyroid Extract against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 808 medications checked with no known interaction — see the full Thyroid Extract list.

See the full Thyroid Extract interactions page
See every supplement checked against Pseudoephedrine, Terfenadine, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
Thyroid Extract monograph › Uses, safety, and every drug interaction in one place Pseudoephedrine, Terfenadine supplement interactions › Every supplement checked against Pseudoephedrine, Terfenadine, ranked by severity Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Thyroid Extract and Pseudoephedrine, Terfenadine Together: Common Questions

Can you take Thyroid Extract with Pseudoephedrine, Terfenadine?
There is only a minor interaction, which is usually not serious. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Thyroid Extract and Pseudoephedrine, Terfenadine interaction?
We rate it Minor. This interaction is generally not clinically significant, but it remains worth noting and discussing with your pharmacist.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Still Have a Question About Pseudoephedrine, Terfenadine and Thyroid Extract?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.