Does Soy Affect Thyroid Function? What the Studies Show

If you have ever typed "does soy affect thyroid" into a search bar late at night, you are in good company. It is one of the most persistent worries attached to soy, it has a real scientific origin story, and it has a surprisingly clear answer once you read the trials instead of the headlines. So let's do exactly that.

Last reviewed: September 2026 by the Haelan Publishing Team

Soy is the seed of the legume Glycine max, and the compounds at the center of the thyroid question are its isoflavones: genistein, daidzein and glycitein, plant polyphenols that occur naturally in soybeans and in every food made from them, including fermented ones like miso, natto, tempeh and Haelan 951.

Does soy affect thyroid function?

In healthy adults who get enough iodine, the best available evidence says soy does not meaningfully change thyroid function. The most rigorous look at the question is a 2019 systematic review and meta-analysis by Otun and colleagues in Scientific Reports, which pooled 18 randomized controlled trials of soy or isoflavone supplementation [1]. Free T4 did not change (a difference of 0.003 pmol/L, statistically indistinguishable from zero), and neither did free T3. TSH, the pituitary signal that tells the thyroid to work harder, rose by an average of 0.248 mIU/L, which the authors called modest and of unclear clinical significance.

A 2006 review in the journal Thyroid by Messina and Redmond examined 14 clinical trials and found that, with a single exception, they showed either no effect or only very modest changes, providing "little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function" [2]. Notice the two conditions in that sentence: "euthyroid" (a normally functioning thyroid) and "iodine-replete" (enough iodine). Most of the rest of this article is about what happens when one of them is not met.

Why did the soy and thyroid worry start?

The concern is real chemistry, but it comes from test tubes, rats and 1960s infant formula rather than from adults eating soy foods. In cell and animal studies, soy isoflavones inhibit thyroid peroxidase, the enzyme that attaches iodine to thyroid hormone as it is being built. A 2002 paper by Doerge and Sheehan in Environmental Health Perspectives reported that genistein accumulated in rat thyroid tissue and reduced thyroid peroxidase activity by up to 80 percent [6]. That is why soy carries the label "goitrogen," a substance that can interfere with how the thyroid uses iodine, and why the NIH Office of Dietary Supplements lists it alongside cassava and cruciferous vegetables [8].

Here is the part that usually gets left out. In those same rats, serum T3, T4 and TSH, thyroid weight and thyroid tissue were all normal, even with the enzyme partly inhibited. The authors concluded that something beyond genistein is needed for overt thyroid toxicity, and that something was iodine deficiency, which greatly increased soy's antithyroid effect while added iodine offset it [6].

I like to think of thyroid peroxidase as a workbench where iodine gets bolted onto thyroid hormone. Genistein is a coworker leaning on the bench. In a test tube, that slows the work. In a body with a steady iodine supply, the bench has spare capacity and the finished hormone rolls out on schedule. Cut the iodine supply and the leaning coworker suddenly matters. The human history fits: reports of underactive thyroids in infants on soy formula stopped once iodine was added to formula in the 1960s, the Linus Pauling Institute notes [7]. If you have read our piece on whether soy raises estrogen, the shape will feel familiar: a plausible mechanism, dramatic animal data, quieter human trials.

Does iodine change whether soy affects the thyroid?

Yes, iodine is the single most important variable here. Every reassuring conclusion above is conditional on adequate iodine, and every worrying observation involves iodine that was marginal or missing. The NIH fact sheet is explicit that goitrogen-containing foods matter mainly for people with marginal iodine status, particularly in iodine-deficient regions, and that reasonable amounts pose no concern for people with adequate intake; the adult recommended dietary allowance is 150 micrograms a day [8]. Messina and Redmond made the same point in 2006, advising soy consumers to make sure their iodine intake is adequate [2].

One practical wrinkle: plant-based beverages used as milk substitutes, including soy beverages, contain relatively little iodine compared with dairy milk [8]. Someone who swaps all their dairy for soy milk and uses non-iodized specialty salt could drift toward the low end without noticing. Seaweed, seafood, eggs, dairy and iodized salt are the usual sources.

Can you eat soy if you take thyroid medication?

People taking levothyroxine generally can eat soy, but the timing question deserves a conversation with the prescriber. The concern is absorption of the pill, not the thyroid gland itself. A 2023 narrative review in Endocrine Practice by Gatta and colleagues screened 213 publications and analyzed the seven that addressed this directly [5]. Case reports had suggested interference [7], but the only prospective randomized crossover study found no difference in levothyroxine absorption when the medication and soy isoflavones were taken together, and the authors concluded that any interference, if present, "seems to have little clinical impact" in adults, while adding that the evidence is thin and the question is not fully settled [5].

Infants are the clearer case: babies with congenital hypothyroidism fed soy formula have required higher levothyroxine doses in published reports [7]. For adults, the standard advice is to take levothyroxine consistently, on an empty stomach, separated from food, coffee, calcium and iron; soy fits into that routine rather than needing a rule of its own. If you want to add fermented soy to your day, here is a sensible way to bring it up:

1. Do not change your medication or its timing on your own.
2. Tell your clinician what you plan to add and roughly when in the day you would have it.
3. Keep the gap between your pill and your first food consistent from day to day, whatever that gap is.
4. Ask whether a follow-up thyroid blood test makes sense after the change.

A quick note before we go further: this article is for educational purposes only and is not medical advice. Thyroid care is individual, so please talk with your healthcare professional about what is right for you, especially if you take thyroid medication.

What about people whose thyroid is already under strain?

This is the one group where soy trials have raised a flag. In a 2011 randomized, double-blind crossover trial in the Journal of Clinical Endocrinology & Metabolism, Sathyapalan and colleagues gave 60 patients with subclinical hypothyroidism (a mildly elevated TSH with normal thyroid hormone levels) 30 grams of soy protein with either 2 mg or 16 mg of isoflavones for eight weeks each [3]. During the 16 mg phase, six female participants progressed to overt hypothyroidism, a 3.6-fold higher rate than during the low-dose phase.

The same group published a follow-up in Frontiers in Endocrinology in 2018, giving 44 participants with subclinical hypothyroidism a much larger 66 mg isoflavone dose against placebo for eight weeks each [4]. TSH, free T4 and free T3 showed no significant difference between phases. Two participants progressed after the isoflavone phase, but the authors noted a high dropout rate, more men than in the earlier study and lower baseline TSH values. The 2019 meta-analysis likewise found its small TSH signal concentrated in studies of people with compromised thyroid function at baseline [1]. The fair reading: if your thyroid is already working near its limit, concentrated isoflavone doses are worth discussing with your endocrinologist.

Study	Who and what	What it found
Otun et al., 2019, meta-analysis [1]	18 randomized trials, mixed populations	No change in free T4 or T3; TSH up 0.248 mIU/L, clinical meaning unclear
Messina & Redmond, 2006, review [2]	14 clinical trials in adults	Little evidence of adverse effects in iodine-replete adults with normal thyroid function
Sathyapalan et al., 2011, RCT [3]	60 adults with subclinical hypothyroidism, 2 vs 16 mg isoflavones	Six participants progressed to overt hypothyroidism during the 16 mg phase
Sathyapalan et al., 2018, RCT [4]	44 adults with subclinical hypothyroidism, 66 mg vs placebo	No significant change in TSH, free T4 or free T3
Gatta et al., 2023, narrative review [5]	7 studies on soy and levothyroxine absorption	Interference, if any, has little clinical impact in adults; evidence limited

Is fermented soy different for the thyroid?

We do not know from direct trials, because almost none of the thyroid research used fermented soy foods; the studies above used soy protein isolate or isoflavone extracts, not a bowl of miso or a glass of fermented soy beverage. What fermentation clearly changes is the chemical form of the isoflavones. In the soybean they are mostly bound to a sugar molecule (the glucoside form); digestion or fermentation releases that sugar, leaving the free aglycone form [7]. We walk through why that matters in fermented vs. unfermented soy, and in our post on equol producers we explain why the same isoflavone can behave differently from one person's gut to the next.

What fermentation does not do is remove the isoflavones or add iodine, so the iodine rule applies to fermented soy exactly as it applies to tofu. Fermentation adds one consideration of its own: fermented soy foods contain tyramine, which is why the Linus Pauling Institute advises people taking MAO inhibitor medications to avoid them [7]. That has nothing to do with the thyroid, but it belongs in any honest list. We make the case in soy isoflavone supplements vs. whole soy foods that whole foods deliver isoflavones at lower doses than extracts, but it would be overreaching to say fermented soy has been shown to be easier on the thyroid. It has not been tested that way.

Who should talk to a clinician before adding soy?

Most people do not need to do anything special. A few groups have a specific, sourced reason to check first.

Situation	Why it matters	Source
Taking levothyroxine or other thyroid hormone	Possible effect on absorption; keep timing consistent and tell your prescriber	[5], [7]
Subclinical hypothyroidism or low iodine intake	Trials in this group showed a TSH signal at higher isoflavone doses; iodine adequacy is the key variable	[3], [8]
Taking tamoxifen or another SERM, or a hormone-sensitive condition	Isoflavones interact with estrogen receptors; seek medical advice, particularly about concentrated soy products	[7]
Taking warfarin	A case report linked large daily soy milk intake with a change in INR readings	[7]
Taking an MAO inhibitor	Fermented soy contains tyramine	[7]
Pregnancy, or feeding an infant	Iodine needs are higher in pregnancy; formula choice affects thyroid medication dosing in infants who need it	[8], [7]
Soy allergy	Fermented soy is still soy	General guidance

What the evidence does and does not show. The reassuring findings come from randomized trials, the strongest evidence for a question like this, but the trials were small, most lasted weeks rather than years, and the 2019 meta-analysis reported high heterogeneity between them [1]. The worrying findings come from animal studies, case reports, and two trials in people whose thyroid was already under strain, using concentrated isoflavone doses. No trial has specifically tested fermented soy foods against thyroid outcomes, and none ran long enough to say what decades of daily soy intake do. Where observational data exist, they show associations, not cause and effect.

Common questions

Is soy a goitrogen?
Yes, in the technical sense. The NIH Office of Dietary Supplements lists soy, along with cassava and cruciferous vegetables, as a food high in goitrogens, compounds that can interfere with how the thyroid uses iodine. The same fact sheet notes this matters mainly for people with marginal iodine status, and that reasonable amounts pose no concern for people with adequate iodine intake [8].

Does soy raise TSH?
Slightly, on average, in pooled trial data. The 2019 meta-analysis of 18 randomized trials found TSH rose by 0.248 mIU/L with soy supplementation, while free T4 and free T3 did not change. The authors described the rise as modest and its clinical significance, if any, as unclear. The signal was concentrated in studies of people whose thyroid function was already compromised at baseline [1].

How long after taking levothyroxine can I eat soy?
There is no soy-specific interval established in the research. A 2023 review found the only randomized crossover trial showed no difference in absorption when levothyroxine and soy isoflavones were taken together, and concluded any effect appears clinically small in adults [5]. The general rule for levothyroxine is consistent timing on an empty stomach, and your prescriber can tailor that to you.

Is fermented soy better for the thyroid than unfermented soy?
No study has tested that directly. Fermentation converts isoflavone glucosides to aglycones and changes how the food is digested [7], but it does not remove isoflavones or add iodine, so the iodine consideration is the same. Fermented soy also contains tyramine, which is relevant for people taking MAO inhibitors, though not for thyroid function itself.

Do I need extra iodine if I eat a lot of soy?
Not extra, but adequate. The adult RDA is 150 micrograms a day [8], and the 2006 review specifically advised soy consumers to make sure their iodine intake meets it [2]. If you have replaced dairy with soy beverages and use non-iodized salt, it is worth asking your clinician whether your intake is where it should be.

The bottom line

Does soy affect thyroid function? For most people, with a healthy thyroid and an ordinary iodine intake, the trials say no in any way that shows up in the numbers that matter. The old worry came from real biochemistry that depends on iodine deficiency to become meaningful. The situations that deserve a conversation are specific: thyroid medication timing, low iodine intake, and a thyroid already under strain, particularly with concentrated isoflavone doses. Fermented soy has not been studied separately on this question, so we will not pretend it has.

Haelan 951 is a concentrated organic, non-GMO fermented soy beverage made with a proprietary nitrogen fermentation process, classified by the FDA as a food rather than a supplement or drug. It contains the naturally occurring isoflavones, saponins and phytosterols found in soy. If you take thyroid medication or have a thyroid condition, the same advice applies to Haelan 951 as to any soy food: bring it up with your clinician, and follow the serving guidance on the label. Read more on the Haelan 951 story page, or explore the wider research in isoflavones and healthy aging.

Discover Haelan 951 →

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References: [1] Otun J, Sahebkar A, Östlundh L, et al. Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function. Scientific Reports, 2019.  [2] Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid, 2006.  [3] Sathyapalan T, et al. The effect of soy phytoestrogen supplementation on thyroid status and cardiovascular risk markers in patients with subclinical hypothyroidism: a randomized, double-blind, crossover study. Journal of Clinical Endocrinology & Metabolism, 2011.  [4] Sathyapalan T, Dawson AJ, Rigby AS, et al. The Effect of Phytoestrogen on Thyroid in Subclinical Hypothyroidism: Randomized, Double Blind, Crossover Study. Frontiers in Endocrinology, 2018.  [5] Gatta E, Maltese V, Cavadini M, et al. Interference or Noninterference Between Soy and Levothyroxine: That Is the Question. A Narrative Review of Literature. Endocrine Practice, 2023.  [6] Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environmental Health Perspectives, 2002.  [7] Linus Pauling Institute, Oregon State University. Soy Isoflavones. Micronutrient Information Center.  [8] National Institutes of Health, Office of Dietary Supplements. Iodine: Fact Sheet for Health Professionals. Updated 2024.

This content is for educational purposes only and is not intended to diagnose, treat, cure, or prevent any disease. Always consult your healthcare professional before making changes to your diet or health routine.