Quick answer

Ask your pharmacist or doctor before combining. The levothyroxine label says dietary fiber may bind the medicine and lower how much is absorbed. It does not name inulin, but FDA lets inulin count as dietary fiber on US labels, and the sources checked include no study of inulin taken with levothyroxine.^1,2

Key facts

The levothyroxine label says soybean flour, cottonseed meal, walnuts and dietary fiber may bind levothyroxine and decrease its absorption.¹
The label does not name inulin. FDA allows inulin to be counted in the dietary fiber line of Supplement Facts labels.²
MedlinePlus says foods and drinks with soybeans, walnuts and dietary fiber may affect how levothyroxine works and asks people to talk to their doctor first.³
A 2021 systematic review found limited evidence about fiber and levothyroxine, from studies of fibers such as bran and psyllium rather than inulin.⁴
Inulin can appear on ingredient lists as chicory root fiber, oligofructose or fructooligosaccharide, so the word inulin may not be on the label.⁵

Ask your pharmacist or doctor first

The levothyroxine label says dietary fiber may bind levothyroxine and lower how much is absorbed. Inulin counts as dietary fiber on US labels, although the levothyroxine label does not name it.^1,2

On the medicine's label

Does dietary fiber affect levothyroxine?

MedlinePlus describes levothyroxine as a hormone that replaces the thyroid hormone the body would normally make, taken for an underactive thyroid.³

The label's food warning is specific. Soybean flour, cottonseed meal, walnuts and dietary fiber may bind levothyroxine in the gut and decrease its absorption. In its pharmacology section the label adds that dietary fiber decreases the bioavailability of the medicine and that absorption is raised by fasting. The label directs taking the medicine on an empty stomach.^1,6

MedlinePlus puts it in plain terms: foods and beverages that contain soybeans, walnuts and dietary fiber may affect how levothyroxine works, and it asks people to talk to their doctor before eating or drinking them.³

Is inulin counted as dietary fiber?

FDA counts inulin and inulin-type fructans as dietary fiber on Nutrition and Supplement Facts labels, a decision it announced in June 2018 while it prepares a rule.^2,5

So an inulin product adds to the fiber number on its label, even though the levothyroxine label says only 'dietary fiber' and names no particular fiber. FDA's science review describes inulin as a soluble fiber that, unlike many soluble fibers, does not thicken into a gel.^2,5

Ingredient lists may show inulin as inulin, chicory root, chicory root extract, chicory root fiber, chicory vegetable fiber, oligofructose or fructooligosaccharide.⁵

That list matters because the word inulin may not appear on the front of a bar, drink or gummy. Check the ingredient list and the dietary fiber line.

What have studies found about fiber and levothyroxine?

A 2021 systematic review of food and supplement interactions with levothyroxine found limited evidence that fiber lowers absorption. It describes a case series of 13 people whose thyroid medicine worked less well after they ate fiber-enriched products such as whole wheat bread, bran, granola and psyllium. It also describes a test in 8 volunteers in which 3.4 g of psyllium lowered absorption of a 600 mcg levothyroxine dose from about 89% to 80%, a change its authors called clinically insignificant.⁴

A 2017 review reached a similar view, saying the effect of dietary fiber on levothyroxine absorption is not yet fully understood and needs more research.⁷

None of these studies tested inulin. The sources checked for this page include no study of inulin taken with levothyroxine, so what it does in practice is not established either way.

Why does a small change in absorption matter with levothyroxine?

The label says levothyroxine has a narrow therapeutic index, meaning overtreatment or undertreatment may have negative effects, and that its dose depends on factors including other medicines and the food taken with it. Prescribers follow thyroid blood tests to adjust it. A 2017 review adds that because the medicine is taken in microgram amounts, interactions in absorption can be clinically significant.^6,7

What to ask your pharmacist about inulin and levothyroxine

Does my fiber supplement, bar, drink or gummy contain inulin or chicory root?
Does it matter when I take levothyroxine in relation to my fiber product or my breakfast?
If I start, stop or change a fiber product, should my thyroid tests be rechecked?
Do I take other products the label names, such as iron, calcium or antacids?

Questions people ask

Can I take inulin with levothyroxine?

The levothyroxine label warns that dietary fiber may lower absorption, and inulin counts as dietary fiber on US labels. The sources checked include no study of inulin itself, so ask your pharmacist or doctor before combining them.^1,2

Does inulin interfere with levothyroxine absorption?

No study in the sources we checked measured it. The label's warning is about dietary fiber in general, and the studies reviewed in 2021 looked at fibers such as bran and psyllium.^1,4

Can I take fiber supplements with levothyroxine?

The label and MedlinePlus both name dietary fiber as something that may affect how levothyroxine works. Ask your pharmacist about your specific fiber product and how to fit it around your medicine.^1,3

What else can you not take with levothyroxine?

The levothyroxine label also names iron and calcium supplements and antacids as products that can decrease its absorption. See the pages on those supplements for what NIH and MedlinePlus say.⁶

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.