Drug & supplement interaction

Phosphate Salts + Levothyroxine

Levothyroxine brand names: Euthyrox, Levothroid, Unithroid, Synthroid, Levoxyl, and others.

No Known Interaction
The bottom line No known interaction

We found no known interaction between Phosphate Salts and Levothyroxine in our sources. Absence of a documented interaction is not the same as proven safety, and does not guarantee one does not exist. Do not start, stop, or change a medication or supplement based on this page; confirm with your pharmacist or doctor first. Additionally, although there is no interaction, the label for Levothyroxine states a food rule worth keeping to, and the two are separately associated with some of the same side effects, which would make a new symptom harder to attribute to one or the other.

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Interaction report

Phosphate Salts and Levothyroxine

We checked these two against our interaction database. Here is what we found.

No Known Interaction Found

We did not find a documented interaction between Levothyroxine and Phosphate Salts in our sources. This does not guarantee that the combination is completely free of risk — there is often limited research on supplements and herbal products, especially around drug interactions.

If you decide to take both, the practical routine is simple: add one new thing at a time and note the date you started it. If the combination is going to disagree with you, it usually shows in the first days to a couple of weeks — and a start date turns “I’ve been feeling off” into something a pharmacist can actually work with. Do not stop a prescribed medicine on your own.

Always talk with your pharmacist or doctor before making changes to your medications or supplements, particularly if you take several medications or have an ongoing health condition.

Phosphate Salts safety at a glance

Phosphate Salts overall Use caution Generally tolerated short-term in healthy people, but excess can harm the kidneys, heart, and mineral balance.
Pregnancy Likely Safe When used orally and appropriately at the recommended dietary allowance (RDA) of 1250 mg daily for individuals 14-18 years of age and 700 mg daily for those ove... Read the full pregnancy detail
Breastfeeding Likely Safe When used orally and appropriately at the recommended dietary allowance (RDA) of 1250 mg daily for individuals 14-18 years of age and 700 mg daily for those ove... Read the full breastfeeding detail

Pregnancy & breastfeeding ratings are from Natural Medicines (Therapeutic Research Center) — the full detail is in the Special populations card below. Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

Still wondering about combining Levothyroxine and Phosphate Salts? Ask Dr. Brian Staiger, PharmD, BCPS directly ›
How we checked

Why No Interaction Is Listed for Levothyroxine

Interactions in our licensed database are recorded against drug classes, not just single drug names. Levothyroxine is filed under 3 classifications — including thyroid hormone, oral drugs. Phosphate Salts has documented interactions with 3 drug classes — the most serious being erdafitinib (balversa) (major), futibatinib (lytgobi) (major) and bisphosphonates (moderate) — and none of Levothyroxine’s classifications are among them.

Classification data comes from our licensed Natural Medicines dataset. A medication can belong to classes beyond those recorded there, so this describes how the check was run — it is not a safety guarantee.

Keep it in perspective

Phosphate Salts Does Interact With Other Medications

Phosphate Salts has 2 major, 10 moderate known drug interactions in our database — Levothyroxine isn’t one of them. If you take anything else, check it against the full list below.

Taking anything else?

Check Another of Your Medications Against Phosphate Salts

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

What to actually watch for

No Interaction Found — But Data Suggests Phosphate Salts and Levothyroxine May Share Several of the Same Side Effects

This is not an interaction. Neither one changes how your body handles the other, and nothing here says they react together. What our sources do show is that each has separately been linked to the effects below — so taking both can make those effects more likely to turn up, and harder to pin on one or the other if they do. One of them is worth acting on rather than waiting out — marked below.

  • A severe allergic reaction Worth acting on Additional detail

    Phosphate Salts

    “Orally, a case of allergic acute coronary syndrome (i.e., Kounis syndrome) is reported in a 43-year-old female after ingesting a specific sodium phosphate laxative product (Travad oral). She presented with maculopapular rash that progressed to anaphylaxis and a non-ST elevation acute coronary syndrome. The patient recovered after hospitalization for 3 days with medical management.”

    Phosphate Salts · Natural Medicines adverse-effect record, reported in myocardial infarction (mi).

    1 source cited
    1. 112894. Zuluaga-Gómez M, González-Arroyave D, Ardila CM. Kounis Syndrome Secondary to Laxative Administration. Case Rep Med 2022;2022:6087176. PubMed

    Levothyroxine

    Listed in the ADVERSE REACTIONS section of the FDA label for Levothyroxine. Read the label on DailyMed ›

  • Trouble sleeping / Tiredness / Headache Additional detail

    Phosphate Salts

    “Orally, phosphate salts can commonly cause malaise. Headaches and sleep disturbance may also occur.”

    Phosphate Salts · Natural Medicines adverse-effect record, reported in insomnia.

    3 sources cited
    1. 93846. Ell C, Fischbach W, Layer P, Halphen M. Randomized, controlled trial of 2 L polyethylene glycol plus ascorbate components versus sodium phosphate for bowel cleansing prior to colonoscopy for cancer screening. Curr Med Res Opin. 2014 Dec;30(12):2493-503. PubMed
    2. 93848. Jung YS, Lee CK, Kim HJ, Eun CS, Han DS, Park DI. Randomized controlled trial of sodium phosphate tablets vs polyethylene glycol solution for colonoscopy bowel cleansing. World J Gastroenterol. 2014 Nov 14;20(42):15845-51. PubMed
    3. 93851. Manukyan MN, Tolan K, Severge U, Attaallah W, Kebudi A, Cingi A. Prospective randomized comparison of oral sodium phosphate and sennoside A+B calcium lavage for colonoscopy preparation. Surg Laparosc Endosc Percutan Tech. 2011 Apr;21(2):90-3. PubMed

    Levothyroxine

    Listed in the ADVERSE REACTIONS section of the FDA label for Levothyroxine. Read the label on DailyMed ›

  • Stomach pain / Diarrhea / Vomiting / Nausea Additional detail

    Phosphate Salts

    “Orally, phosphate salts can cause gastrointestinal irritation, nausea, abdominal pain, bloating, anal irritation, and vomiting (15,2494,2495,2496,2497,93846,93848,93850,93851,93853,107008). Sodium and potassium phosphates can cause diarrhea (15). Aluminum phosphate can cause constipation (15).”

    Phosphate Salts · Natural Medicines adverse-effect record, reported in constipation.

    6 sources cited
    1. 15. McEvoy GK, ed. AHFS Drug Information. Bethesda, MD: American Society of Health-System Pharmacists, 1998.
    2. 2494. Hill AG, Teo W, Still A, et al. Cellular potassium depletion predisposes to hypokalaemia after oral sodium phosphate. Aust N Z J Surg 1998;68:856-8. PubMed
    3. 2495. Clarkston WK, Tsen TN, Dies DF, et al. Oral sodium phosphate versus sulfate-free polyethylene glycol electrolyte lavage solution in outpatient preparation for colonoscopy: a prospective comparison. Gastrointest Endosc 1996;43:42-8. PubMed
    4. 2496. Fine A, Patterson J. Severe hyperphosphatemia following phosphate administration for bowel preparation in patients with renal failure: two cases and a review of the literature. Am J Kidney Dis 1997;29:103-5. PubMed
    5. 2497. DiPalma JA, Buckley SE, Warner BA, et al. Biochemical effects of oral sodium phosphate. Dig Dis Sci 1996;41:749-53. PubMed
    6. 93846. Ell C, Fischbach W, Layer P, Halphen M. Randomized, controlled trial of 2 L polyethylene glycol plus ascorbate components versus sodium phosphate for bowel cleansing prior to colonoscopy for cancer screening. Curr Med Res Opin. 2014 Dec;30(12):2493-503. PubMed

    Levothyroxine

    Listed in the ADVERSE REACTIONS section of the FDA label for Levothyroxine. Read the label on DailyMed ›

  • A skin rash Additional detail

    Phosphate Salts

    Recorded for Phosphate Salts in the same Natural Medicines adverse-effect record shown under A severe allergic reaction above.

    Levothyroxine

    Listed in the ADVERSE REACTIONS section of the FDA label for Levothyroxine. Read the label on DailyMed ›

Reported for Phosphate Salts but not commonly for Levothyroxine: an electrolyte imbalance, kidney problems, constipation and high potassium.

Effects for Levothyroxine come from the ADVERSE REACTIONS section of its FDA label; effects for Phosphate Salts come from the Natural Medicines adverse-effect record, quoted above. Wording is matched across sources by our own clinical vocabulary, so treat this as a prompt to pay attention, not a diagnosis.

Special populations

Phosphate Salts in Pregnancy & Breastfeeding

Pregnancy And Lactation Likely Safe

When used orally and appropriately at the recommended dietary allowance (RDA) of 1250 mg daily for individuals 14-18 years of age and 700 mg daily for those over 18 years of age. ...when sodium phosphate is used rectally and appropriately short-term. ...when used intravenously. Intravenous potassium phosphate is an FDA-approved prescription drug.

Pregnancy And Lactation Possibly Unsafe

When phosphate (expressed as phosphorus) intake exceeds the tolerable upper intake level (UL). Hyperphosphatemia, resulting in electrolyte disturbances, alterations in calcium homeostasis, and calcification of nonskeletal tissues, may occur. The UL during pregnancy is 3.5 grams daily. During lactation, the UL is 4 grams daily.

DISCLAIMER: This tool is intended for informational purposes only, and should not be interpreted as specific medical advice. Patients should consult with a qualified healthcare provider before making decisions about therapies and/or health conditions.© 2026 Therapeutic Research Center

How to take them

Some Levothyroxine Products Have a Food Rule — Phosphate Salts Should Not Break It

At least one FDA label for a product containing Levothyroxine says to take it on an empty stomach. Most supplements — Phosphate Salts included — are taken with a meal, so the easy mistake is swallowing both together at breakfast. Where that rule applies, keep Levothyroxine on its own and move Phosphate Salts to a meal later in the day.

This may not be true of your version. Food instructions belong to the product, not the ingredient: two Levothyroxine products can carry different directions, and extended-release forms often differ from immediate-release ones. The label we read is quoted below — check the directions on your own box or bottle, or ask your pharmacist, for how to take the exact product you were given.

A food rule is not an interaction — it is about how much of the medicine you absorb. The directions that came with your own product, and your pharmacist, outrank anything on this page.

Overlapping uses

What Levothyroxine Is Prescribed For, and What Phosphate Salts Has Been Studied For

These two come up together because they touch the same health topic — not because they do the same job. Levothyroxine is a prescription medicine with approved uses on its FDA label. Phosphate Salts is a supplement: what follows is what researchers have looked at it for, and how strong that evidence is.

Levothyroxine is prescribed for: Levothyroxine replaces or supplements the thyroid hormone your body isn't making enough of. It's used for hypothyroidism — an underactive thyroid — in adults, children, and even newborns, whether the cause is the thyroid gland itself, the pituitary gland, or the hypothalamus in the brain. It's also used alongside surgery and radioiodine treatment in people with certain types of thyroid cancer, to help suppress the hormone that can stimulate cancer growth. It is not meant for weight loss, treating benign thyroid nodules in people who have enough iodine, or for hypothyroidism during recovery from a temporary condition called subacute thyroiditis.

Phosphate Salts has been studied for:

  • Constipation Natural Medicines rates the evidence for Phosphate Salts: Likely Effective Read what the research shows

    Phosphate Salts for constipation

    Oral or rectal sodium phosphate is approved by the US Food and Drug Administration (FDA) as an ingredient in over-the-counter (OTC) products to treat constipation.

Effectiveness ratings and the research summaries above are the verbatim record from the Natural Medicines database, which grades evidence from “Effective” down to “Insufficient Reliable Evidence To Rate”. A rating below the top of that scale means researchers have looked at it — not that it works, and never that it can stand in for a prescribed medicine. Do not stop or replace Levothyroxine on your own; if you are taking Phosphate Salts for the same health topic, tell your prescriber, especially if the medicine is being adjusted to a target.

From our Q&A

Questions About Phosphate Salts or Levothyroxine

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Phosphate Salts or Levothyroxine, 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 Levothyroxine

This page covers Phosphate Salts only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Levothyroxine page lists 50 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Tiratricol (major), Acetyl-l-carnitine (moderate), Alpha-lipoic Acid (moderate) and Ascophyllum Nodosum (moderate).

By severity: 2 major · 39 moderate · 9 minor.

From our pharmacist’s overview on the Levothyroxine page

“Levothyroxine replaces a hormone your body measures in tiny amounts, so small shifts in its blood level show up in how you feel, and I treat this list with genuine care. Nearly everything on it does one of two things: blocks the pill from being absorbed in the gut, the way everyday minerals like calcium do by binding it into complexes,…”

Reviewed by 2 · Jul 18, 2026

Open the Levothyroxine interaction page

One more smart check

Taking Phosphate Salts as Part of a Dietary Supplement Product? Check It for Interactions

We hold 6 real supplement products containing Phosphate Salts, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Phosphate Salts. Even though Phosphate Salts itself has no known interaction with Levothyroxine, another ingredient on your label might. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Phosphate Salts and Levothyroxine 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 Phosphate Salts
Steady interest

#1,142 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Levothyroxine
Top 10

#2 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.

Where does our data come from?
  • Source: the Natural Medicines (TRC Healthcare) clinical database — the evidence-graded professional reference for herb & supplement interactions.
  • Citation library: 103,510 references behind our interaction content, 65,311 of them resolved to verified PubMed/DOI sources. See every Phosphate Salts reference on this page ↓
  • Freshness: we automatically re-check this combination every week — this pair was last verified September 13, 2026. The Phosphate Salts and Levothyroxine records this page draws on were last updated July 10, 2026.
  • What “no known interaction” means: no documented interaction in the sources above — not a guarantee of safety. Supplements are studied far less than prescription drugs, so we say exactly what the evidence supports, and nothing more.
Data structure & sourcing designed and verified by .
Supplement

About Phosphate Salts

What is Phosphate Salts?

  • Low blood phosphate levels (hypophosphatemia)
  • Bowel preparation/laxative use
  • Athletic performance ('phosphate loading')
  • Preventing certain kidney stones
  • Calcium supplementation balance

Phosphate salts are mineral compounds used mainly to correct low phosphate levels, as laxatives, and sometimes by athletes. Most people who eat a normal diet already get plenty of phosphate and do not need a supplement. Because too much phosphate can be dangerous—especially for people with kidney disease—use these products only under the guidance of a doctor or pharmacist.

Mineral salts (not a plant) · Tablets, capsules, powders, oral solutions, effervescent tablets, and intravenous (medical) solutions

An interaction check asks whether these two react with each other — it does not ask whether Phosphate Salts is a good idea for someone in your situation. Here is what our Phosphate Salts record says about it on its own.

Phosphate Salts: dosing

There is no single standard supplement dose that fits everyone, because the right amount depends on why it is being used—correcting a deficiency, bowel prep, or another reason—and on your kidney function and blood levels.Phosphate products used in hospitals or for bowel prep are dosed by a healthcare provider. For over-the-counter products, follow the label exactly and do not take more than directed.

From our Phosphate Salts monograph — the full record covers uses, side effects and every documented interaction.

Read the full Phosphate Salts monograph & all interactions
Medication

About Levothyroxine

What Is Levothyroxine Used For?

Brand names: Euthyrox, Levothroid, Unithroid, Synthroid, Levoxyl, Levo-T, Eltroxin, Estre, Levothyroxine, Novothyrox, Levolet

Levothyroxine replaces or supplements the thyroid hormone your body isn't making enough of. It's used for hypothyroidism — an underactive thyroid — in adults, children, and even newborns, whether the cause is the thyroid gland itself, the pituitary gland, or the hypothalamus in the brain.

It's also used alongside surgery and radioiodine treatment in people with certain types of thyroid cancer, to help suppress the hormone that can stimulate cancer growth. It is not meant for weight loss, treating benign thyroid nodules in people who have enough iodine, or for hypothyroidism during recovery from a temporary condition called subacute thyroiditis.

See full Levothyroxine drug information
Keep checking

Other Drugs & Phosphate Salts

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

2Major
10Moderate
0Minor
2,817No known interaction
Major

2 interactions

Moderate

10 interactions

No interaction 2,817 drugs checked — show the list
Evidence & sources

Phosphate Salts References

The 23 references that drive our Phosphate Salts monograph and interaction data, from the evidence-graded Natural Medicines (TRC Healthcare) database. Citations with a link open the study on PubMed or the publisher’s site.

  1. McEvoy GK, ed. AHFS Drug Information. Bethesda, MD: American Society of Health-System Pharmacists, 1998.
  2. Hill AG, Teo W, Still A, et al. Cellular potassium depletion predisposes to hypokalaemia after oral sodium phosphate. Aust N Z J Surg 1998;68:856-8. PubMed
  3. Clarkston WK, Tsen TN, Dies DF, et al. Oral sodium phosphate versus sulfate-free polyethylene glycol electrolyte lavage solution in outpatient preparation for colonoscopy: a prospective comparison. Gastrointest Endosc 1996;43:42-8. PubMed
  4. Fine A, Patterson J. Severe hyperphosphatemia following phosphate administration for bowel preparation in patients with renal failure: two cases and a review of the literature. Am J Kidney Dis 1997;29:103-5. PubMed
  5. DiPalma JA, Buckley SE, Warner BA, et al. Biochemical effects of oral sodium phosphate. Dig Dis Sci 1996;41:749-53. PubMed
  6. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press, 1999. Available at: http://books.nap.edu/books/0309063507/html/index.html.
  7. Campisi P, Badhwar V, Morin S, Trudel JL. Postoperative hypocalcemic tetany caused by Fleet Phospho-Soda preparation in a patient taking alendronate sodium. Dis Colon Rectum 1999;42:1499-501. DOI
  8. Ell C, Fischbach W, Layer P, Halphen M. Randomized, controlled trial of 2 L polyethylene glycol plus ascorbate components versus sodium phosphate for bowel cleansing prior to colonoscopy for cancer screening. Curr Med Res Opin. 2014 Dec;30(12):2493-503. PubMed
  9. Jung YS, Lee CK, Kim HJ, Eun CS, Han DS, Park DI. Randomized controlled trial of sodium phosphate tablets vs polyethylene glycol solution for colonoscopy bowel cleansing. World J Gastroenterol. 2014 Nov 14;20(42):15845-51. PubMed
  10. Lee SH, Lee DJ, Kim KM, Seo SW, Kang JK, Lee EH, Lee DR. Comparison of the efficacy and safety of sodium phosphate tablets and polyethylene glycol solution for bowel cleansing in healthy Korean adults. Yonsei Med J. 2014 Nov;55(6):1542-55. PubMed
  11. Manukyan MN, Tolan K, Severge U, Attaallah W, Kebudi A, Cingi A. Prospective randomized comparison of oral sodium phosphate and sennoside A+B calcium lavage for colonoscopy preparation. Surg Laparosc Endosc Percutan Tech. 2011 Apr;21(2):90-3. PubMed
  12. van Vugt van Pinxteren MW, van Kouwen MC, van Oijen MG, van Achterberg T, Nagengast FM. A prospective study of bowel preparation for colonoscopy with polyethylene glycol-electrolyte solution versus sodium phosphate in Lynch syndrome: a randomized trial. F PubMed
  13. Choi NK, Lee J, Chang Y, Kim YJ, Kim JY, Song HJ, Shin JY, Jung SY, Choi Y, Lee JH, Park BJ. Acute renal failure following oral sodium phosphate bowel preparation: a nationwide case-crossover study. Endoscopy. 2014 Jun;46(6):465-70. PubMed
  14. Brunelli SM. Association between oral sodium phosphate bowel preparations and kidney injury: a systematic review and meta-analysis. Am J Kidney Dis. 2009 Mar;53(3):448-56. PubMed
  15. Schaefer M, Littrell E, Khan A, Patterson ME. Estimated GFR Decline Following Sodium Phosphate Enemas Versus Polyethylene Glycol for Screening Colonoscopy: A Retrospective Cohort Study. Am J Kidney Dis. 2016 Apr;67(4):609-16. PubMed

Medication overviews come from MedlinePlus (NIH); supplement label data from the NIH Dietary Supplement Label Database. Content is written and reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

Pharmacist Counseling Corner

Taking Phosphate Salts and Levothyroxine Together: Common Questions

Is it safe to take Phosphate Salts with Levothyroxine?
We do not list a known interaction between Phosphate Salts and Levothyroxine. That does not guarantee the combination is risk-free, so confirm with your pharmacist or doctor — especially if you take other medications or have a health condition.
Do Phosphate Salts and Levothyroxine have any of the same side effects?
Yes — our sources link each of them separately to 9 of the same effects. That is not an interaction: neither changes how your body handles the other. It does mean that if one of those effects turns up, it can be harder to tell which one is behind it. They are listed on this page with the source behind each.
Should I take Phosphate Salts and Levothyroxine at the same time of day?
At least one FDA label for a product containing Levothyroxine says to take it on an empty stomach, and supplements are usually taken with a meal — so taking them together at breakfast is the easy mistake. Where that applies, keep Levothyroxine on its own and move Phosphate Salts to a meal later in the day. Food instructions belong to the product rather than the ingredient, so check the directions on your own box. This is about absorption, not an interaction.
Are Phosphate Salts and Levothyroxine used for the same thing?
They overlap in purpose, which usually means the real question is whether you need both rather than whether they are safe together. This page lists the shared uses and how strong the evidence is for Phosphate Salts in each. That is a conversation for your prescriber, especially if Levothyroxine is being adjusted to a target.
How was the Phosphate Salts and Levothyroxine combination checked?
Interactions in our licensed database are recorded against drug classes. Levothyroxine is filed under 3 classifications, and Phosphate Salts has documented interactions with 3 drug classes — none of Levothyroxine's classifications are among them. A medication can belong to classes beyond those recorded in our dataset, so this describes how the check was run rather than guaranteeing safety.
Does Phosphate Salts interact with any other medications?
Yes. Phosphate Salts has 12 documented drug interactions in our database — Levothyroxine is simply not one of them. If you take anything else, it is worth checking each one; they are all listed on this page.
Does Levothyroxine interact with any supplements?
Yes. Levothyroxine has documented interactions with 50 supplements in our database — Phosphate Salts is not one of them. The Levothyroxine interaction page linked on this page lists every one, ranked by severity.

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

Still Have a Question About Levothyroxine and Phosphate Salts?

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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.