NutriStack
Levothyroxine and Soy Isoflavones, timing-sensitive.
Soy isoflavones (genistein and daidzein) can reduce levothyroxine absorption by binding to the medication in the gastrointestinal tract, forming complexes that are poorly absorbed. Studies show soy can reduce peak T4 levels by approximately 22% and total absorption by nearly 19%. The effect is most clinically significant in infants with congenital hypothyroidism on soy formula and in adults consuming large amounts of soy products near their levothyroxine dose.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
Same shape as the other 1,729 pairs in the public database.
From the interaction database
What the row says.
Every entry follows the same shape: what is happening, the mechanism, the recommendation, and the primary literature.
At a glance
- Substances
- Levothyroxine and Soy Isoflavones
- Pair type
- Timing Sensitive
- Evidence (highest tier)
- Moderate
- Source citations
- 3 sources
- Stack Score effect
- −5 to your Stack Score (per scored timing-sensitive row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Timing Sensitive · Moderate evidence
Timing Sensitive
What is happening. Soy isoflavones (genistein and daidzein) can reduce levothyroxine absorption by binding to the medication in the gastrointestinal tract, forming complexes that are poorly absorbed. Studies show soy can reduce peak T4 levels by approximately 22% and total absorption by nearly 19%. The effect is most clinically significant in infants with congenital hypothyroidism on soy formula and in adults consuming large amounts of soy products near their levothyroxine dose.
Mechanism. Soy isoflavones, primarily genistein and daidzein, form chelation complexes with levothyroxine in the intestinal lumen, reducing its absorption. Soy may also inhibit thyroid peroxidase activity and interfere with iodine uptake. Additionally, soy protein and fiber can adsorb levothyroxine, further reducing bioavailability when taken concurrently.
Recommendation. Separate levothyroxine from soy products and soy isoflavone supplements by at least 4 hours. Take levothyroxine on an empty stomach with water in the morning, and consume soy products later in the day. If TSH levels rise after increasing soy intake, a dose adjustment may be needed. Inform your prescriber about regular soy consumption.
Minimum separation. 240
Sources (3)
- Benvenga S et al. Interference or noninterference between soy and levothyroxine: a narrative review of literature. Endocr Pract. 2024;30(1):73-79. PMID 37924928
- Conrad SC et al. Soy formula complicates management of congenital hypothyroidism. Arch Dis Child. 2004;89(1):37-40. PMID 14709501
- Ross DS. Treating hypothyroidism is not always easy: When to treat subclinical hypothyroidism, TSH goals in the elderly, and alternatives to levothyroxine monotherapy. Journal of Internal Medicine. 2022. PMID 34766382
Stack Score
How this pair moves the number.
Effect on the composite score
If both Levothyroxine and Soy Isoflavones are in the same stack, this pair applies −5 to your Stack Score (per scored timing-sensitive row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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