NutriStack
Calcium and Levothyroxine, timing-sensitive.
Calcium supplements significantly reduce levothyroxine absorption by 20-25% through chelation in the gastrointestinal tract. This can lead to subtherapeutic thyroid hormone levels, worsening hypothyroid symptoms, and the need for dose adjustments. The interaction is well-documented in multiple controlled studies.
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
- Calcium and Levothyroxine
- Pair type
- Timing Sensitive
- Evidence (highest tier)
- Strong
- Source citations
- 5 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. Calcium supplements significantly reduce levothyroxine absorption by 20-25% through chelation in the gastrointestinal tract. This can lead to subtherapeutic thyroid hormone levels, worsening hypothyroid symptoms, and the need for dose adjustments. The interaction is well-documented in multiple controlled studies.
Mechanism. Calcium cations form insoluble chelate complexes with levothyroxine in the acidic environment of the stomach, preventing absorption across the intestinal mucosa. The calcium-thyroxine complex is excreted in the feces.
Recommendation. Separate levothyroxine and calcium by at least 4 hours. Take levothyroxine on an empty stomach in the morning, and calcium later in the day. Have TSH monitored when starting or stopping calcium supplements.
Minimum separation. 240
Sources (5)
- Singh N et al. Effect of calcium carbonate on the absorption of levothyroxine. JAMA. 2000;283(21):2822-2825. PMID 10838651
- Zamfirescu I, Carlson HE. Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid. 2011;21(5):483-486. PMID 21595514
- 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
- Liu C, Kuang X, Li K, Guo X, Deng Q, Li D. Effects of combined calcium and vitamin D supplementation on osteoporosis in postmenopausal women: a systematic review and meta-analysis of randomized controlled trials. Food & Function. 2020. PMID 33237064
- Yao P, Bennett D, Mafham M et al.. Vitamin D and Calcium for the Prevention of Fracture: A Systematic Review and Meta-analysis. JAMA Network Open. 2019. PMID 31860103
Stack Score
How this pair moves the number.
Effect on the composite score
If both Calcium 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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