NutriStack
Iron and Levothyroxine, timing-sensitive.
Iron supplements form an insoluble complex with levothyroxine in the GI tract, reducing thyroid hormone absorption by up to 75% in some studies. This is one of the most clinically significant absorption interactions with levothyroxine and frequently leads to treatment failure if timing is not managed.
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
- Iron and Levothyroxine
- Pair type
- Timing Sensitive
- Evidence (highest tier)
- Strong
- Source citations
- 7 sources
- Stack Score effect
- −5 to your Stack Score (per scored timing-sensitive row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Timing Sensitive · Strong evidence
Timing Sensitive
What is happening. Iron supplements form an insoluble complex with levothyroxine in the GI tract, reducing thyroid hormone absorption by up to 75% in some studies. This is one of the most clinically significant absorption interactions with levothyroxine and frequently leads to treatment failure if timing is not managed.
Mechanism. Ferrous and ferric iron ions form insoluble, non-absorbable chelate complexes with thyroxine in the stomach and duodenum. The iron-thyroxine complex passes through the GI tract without absorption.
Recommendation. Separate levothyroxine and iron supplements by at least 4 hours. Take levothyroxine first thing in the morning on an empty stomach, and iron later in the day. Monitor TSH levels when starting or changing iron supplementation.
Minimum separation. 240
Sources (7)
- Campbell NR et al. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med. 1992;117(12):1010-1013. PMID 1443969
- Shakir KM et al. Ferrous sulfate-induced increase in requirement for thyroxine in a patient with primary hypothyroidism. South Med J. 1997;90(6):637-639. PMID 9191742
- 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
- Fischer JAJ, Cherian AM, Bone JN, Karakochuk CD. The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trials. Nutrition Reviews. 2023. PMID 36728680
- Rehman T, Agrawal R, Ahamed F et al.. Optimal dose and duration of iron supplementation for treating iron deficiency anaemia in children and adolescents: A systematic review and meta-analysis. PLoS One. 2025. PMID 39951396
- Gutema BT, Sorrie MB, Megersa ND et al.. Effects of iron supplementation on cognitive development in school-age children: Systematic review and meta-analysis. PLoS One. 2023. PMID 37368919
- Hansen R, Sejer EPF, Holm C, Schroll JB. Iron supplements in pregnant women with normal iron status: A systematic review and meta-analysis. Acta Obstetricia et Gynecologica Scandinavica. 2023. PMID 37403900
Stack Score
How this pair moves the number.
Effect on the composite score
If both Iron and Levothyroxine 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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