Drug & supplement interaction

Ascophyllum Nodosum + Propylthiouracil, PTU

Propylthiouracil, PTU brand name: Propylthiouracil.

Moderate Interaction
Medically reviewed by · Updated September 2026
The bottom line Moderate

Ascophyllum Nodosum has a moderate interaction with Propylthiouracil, PTU. Due to its iodine content, Ascophyllum nodosum might alter the effects of antithyroid drugs.

Read the full interaction report
SeverityModerate
LikelihoodPossible
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Ascophyllum Nodosum and Propylthiouracil, PTU

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

Moderate

ANTITHYROID DRUGS

Why this applies to Propylthiouracil, PTU

Propylthiouracil, PTU falls under ANTITHYROID DRUGS — the group Ascophyllum Nodosum interacts with.

Due to its iodine content, Ascophyllum nodosum might alter the effects of antithyroid drugs.
Ascophyllum nodosum contains iodine, although the bioavailability of iodine from Ascophyllum nodosum is lower than that of potassium iodide. Iodine in high doses has been reported to cause both hyperthyroidism and hypothyroidism, depending on the individual's past medical history. Taking Ascophyllum nodosum while using antithyroid drugs could alter the effects of the antithyroid drugs.
Severity Moderate Likelihood Possible Evidence D

Interaction 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.

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

Here's the thing to keep in mind: you're taking propylthiouracil (PTU) to calm down an overactive thyroid by blocking how much thyroid hormone your gland makes. Ascophyllum nodosum (brown seaweed or bladderwrack) naturally carries iodine, and iodine is the raw material your thyroid uses to build hormone. So adding a concentrated iodine source while you're trying to dial things down can work against your treatment and make it harder to control your symptoms, like a racing heart, jitteriness, or weight changes.

To be fair, this is mostly a theoretical concern based on how iodine behaves, not strong human studies, and the iodine in this seaweed is absorbed less easily than other forms. Still, because thyroid balance is delicate, I'd skip the supplement and check with your doctor or pharmacist before using it.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Sterling JB, Heymann WR. Potassium iodide in dermatology: a 19th century drug for the 21st century-uses, pharmacology, adverse effects, and contraindications. J Am Acad Dermatol 2000;43:691-7. PubMed
  2. Combet E, Ma ZF, Cousins F, et al. Low-level seaweed supplementation improves iodine status in iodine-insufficient women. Br J Nutr 2014;112:753-61. PubMed
  3. Simmons-Boyce JL, Purcell SL, Nelson CM, MacKinnon SL. Dietary Ascophyllum nodosum increases urinary excretion of tricarboxylic acid cycle intermediates in male Sprague-dawley rats. J Nutr 2009;139:1487-94. PubMed
  4. Sorge US, Henriksen M, Bastan A, et al. Short communication: Iodine concentrations in serum, milk, and tears after feeding Ascophyllum nodosum to dairy cows-A pilot study. J Dairy Sci 2016;99:8472-6. PubMed

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

Ascophyllum Nodosum Also Interacts With Other Medications

Ascophyllum Nodosum has 21 moderate known drug interactions in our database — the Propylthiouracil, PTU one above is one of them. If you take anything else, check it against the full list below.

Moderate 21 No known interaction 2,809 (incl. Propylthiouracil, PTU)
Taking anything else?

Check Another of Your Medications Against Ascophyllum Nodosum

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

From our Q&A

Questions About Ascophyllum Nodosum or Propylthiouracil, PTU

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Ascophyllum Nodosum or Propylthiouracil, PTU, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Propylthiouracil, PTU

This page covers Ascophyllum Nodosum only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Propylthiouracil, PTU page lists 64 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Beer (major), Siberian Cocklebur (major), Wine (major) and Ashwagandha (moderate).

By severity: 4 major · 51 moderate · 9 minor.

Open the Propylthiouracil, PTU interaction page

One more smart check

Taking Ascophyllum Nodosum as Part of a Dietary Supplement Product? Check It for Interactions

We hold 360 real supplement products containing Ascophyllum Nodosum, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Ascophyllum Nodosum. Your product may carry other ingredients that interact with Propylthiouracil, PTU too, on top of Ascophyllum Nodosum — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Ascophyllum Nodosum and Propylthiouracil, PTU 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 Ascophyllum Nodosum
Top half

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

29 days ago today
Medication Propylthiouracil, PTU
Steady interest

#2,353 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 Ascophyllum Nodosum

What is Ascophyllum Nodosum?

Ascophyllum Nodosum
  • Weight management
  • Blood sugar support
  • Thyroid support (iodine source)
  • Digestive health
  • Source of vitamins and minerals
  • Joint comfort

Ascophyllum nodosum (often sold as bladderwrack or brown seaweed) is a nutrient-rich algae that people use for weight, blood sugar, and thyroid support, but solid human evidence is limited for most of these uses. Because it naturally contains iodine and can absorb heavy metals like arsenic, it should be used cautiously, and pregnant or breastfeeding people should avoid it unless a doctor approves.

Read the full Ascophyllum Nodosum monograph & all interactions
Medication

About Propylthiouracil

What Is Propylthiouracil Used For?

Brand name: Propylthiouracil

Propylthiouracil tablets are used to treat hyperthyroidism — a condition where the thyroid gland produces too much hormone. It is specifically approved for patients with Graves' disease or toxic multinodular goiter who cannot tolerate methimazole and for whom surgery or radioactive iodine therapy is not a suitable option.

Propylthiouracil is also used to reduce hyperthyroid symptoms before a patient undergoes thyroid surgery (thyroidectomy) or radioactive iodine therapy, again in patients who cannot tolerate methimazole. It may also be the preferred antithyroid drug during or just before the first trimester of pregnancy.

See full Propylthiouracil drug information
Keep checking

Other Drugs & Ascophyllum Nodosum

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

0Major
21Moderate
0Minor
2,808No known interaction
21 Moderate 2,808 No interaction
Moderate

21 interactions

No interaction 2,808 drugs checked — show the list

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

Showing the 60 most looked-up of 2,808 medications checked with no known interaction — see the full Ascophyllum Nodosum list.

See the full Ascophyllum Nodosum interactions page
See every supplement checked against Propylthiouracil, PTU, 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
Ascophyllum Nodosum monograph › Uses, safety, and every drug interaction in one place Propylthiouracil, PTU supplement interactions › Every supplement checked against Propylthiouracil, PTU, ranked by severity Propylthiouracil drug information › The full plain-English guide to this medication 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 Ascophyllum Nodosum and Propylthiouracil, PTU Together: Common Questions

Can you take Ascophyllum Nodosum with Propylthiouracil, PTU?
There is a moderate interaction, so it may be acceptable with monitoring or timing changes — but check with your pharmacist first. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Ascophyllum Nodosum and Propylthiouracil, PTU interaction?
We rate it Moderate. This combination may require monitoring, a dose adjustment, or separating the times you take each one. Consult your pharmacist before using them together.

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

Still Have a Question About Propylthiouracil, PTU and Ascophyllum Nodosum?

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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.