Absorption of Levothyroxine When Coadministered with Various Calcium Formulations
Open accessAbstract
BACKGROUND: Calcium carbonate is a commonly used dietary supplement and has been shown to interfere with levothyroxine absorption. However, calcium citrate, which is also used for supplementation purposes, has not been studied previously and calcium acetate, which is used to treat hyperphosphatemia in renal failure, has been reported to show little or no interference with levothyroxine absorption in a retrospective pharmacoepidemiologic study. We aimed to compare the effect of these three calcium formulations on levothyroxine absorption. MATERIALS AND METHODS: The study was conducted in eight healthy, euthyroid adults. We performed single-dose pharmacokinetic studies in which we measured levothyroxine absorption when given alone or when coadministered with calcium carbonate, calcium citrate, or calcium acetate in doses containing 500 mg elemental calcium. Serum thyroxine was measured at intervals over a 6-hour period after ingestion of the study drugs. RESULTS: Coadministration of each of the three calcium preparations significantly reduced levothyroxine absorption by about 20%-25% compared with levothyroxine given alone. CONCLUSIONS: Contrary to a prior report, our data suggest that calcium acetate interferes with levothyroxine absorption in a manner similar to that seen with calcium carbonate and calcium citrate. Although the effect of calcium is modest compared with some other medications previously studied, hypothyroid patients should be cautioned to take their levothyroxine well-separated from all of these calcium formulations.
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The list of medications that can alter the absorption of LT4 is extensive and includes calcium carbonate, PPIs, bile acid sequestrants (cholestyramine and colesevelam), phosphate binders, ferrous sulfate, aluminum-containing antacids, and sucralfate.
“The list of medications that can alter the absorption of LT4 is extensive and includes calcium carbonate (132-134), PPIs (135), bile acid sequestrants (cholestyramine and colesevelam) (136,137), phosphate binders (138,139), ferrous sulfate (140,141), aluminum-containing antacids (142,143), and sucralfate (144,145).”
From Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement (2014) · cites this paper as [134]
Calcium carbonate, citrate, and acetate all appear to have similar effects, with absorption studies in healthy volunteers showing a decrement in absorption of approximately 20% (134).
“Calcium carbonate, citrate, and acetate all appear to have similar effects, with absorption studies in healthy volunteers showing a decrement in absorption of approximately 20% (134).”
From Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement (2014) · cites this paper as [134]
Lower L-T4 bioavailability applies to other calcium salts.
“Lower L-T4 bioavailability applies to other calcium salts (43).”
From L-T4 therapy in the presence of pharmacological interferents (2020) · cites this paper as [43]
Ingestion of calcium carbonate supplements is associated with lower total T4 (TT4) levels and elevated TSH levels relative to baseline.
“The ingestion of calcium carbonate supplements is associated with lower total T4 (TT4) levels and elevated TSH levels, relative to baseline [ 65 – 67 ].”
From Factors influencing the levothyroxine dose in the hormone replacement therapy of primary hypothyroidism in adults (2022) · cites this paper as [67]
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References 15 · Cited by 76
Research works printed but not linked: 3
- [12]Lamson MJ. Pamplin CL. Rolleri RL. Klein I. Quantitation of a substantial reduction in levothyroxine (T4) absorption by food. Thyroid. 2004;14:876. (abstract). [Google Scholar]
- [15]National Osteoporosis Foundation 2010 Clinician's Guide to Prevention and Treatment of Osteoporosis. National Osteoporosis Foundation; Washington, DC: [Jun 28;2010 ]. [Google Scholar]
- [16]Fresenius Medical Care North America PhosLo® Prescribing Information; Waltham, MA: [Jun 14;2010 ]. [Google Scholar]
- Authors
- Zamfirescu, I., & Carlson, H. E.
- Affiliations
- Stony Brook School (US)
- ORCID
- Carlson, Harold E.
- Venue
- Thyroid
- Publisher
- SAGE Publications
- Published
- 2011-05
- Volume
- 21
- Issue
- 5
- Pages
- 483-486
- ISSN
- 1050-7256 (ISSN-L)
- 1557-9077
- PMC
- PMC3092723
- OpenAlex
- W1975941823
- Cited by
- 76
- References
- 15
- Citations by year
- 2026: 4 · 2025: 6 · 2024: 4 · 2023: 6 · 2022: 5 · 2021: 4 · 2020: 7 · 2019: 5 · 2018: 7 · 2017: 5 · 2016: 2 · 2015: 7 · 2014: 3 · 2013: 9
- FWCI
- 3.14
- Type
- journal article
- Language
- en
- Open access
- green
- www.ncbi.nlm.nih.gov
- Locations
- Thyroid (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Funders
- Stony Brook University
- National Institutes of Health (M01RR10710)
- National Center for Research Resources (#M01RR10710, M01RR010710)
- Topics
- Health Sciences › Medicine › Endocrinology, Diabetes and Metabolism › Thyroid Disorders and Treatments
- Health Sciences › Medicine › Nephrology › Parathyroid Disorders and Treatments
- Life Sciences › Pharmacology, Toxicology and Pharmaceutics › Pharmacology › Pharmacogenetics and Drug Metabolism
- Keywords
- levothyroxine, drug absorption, calcium carbonate, calcium citrate, calcium acetate, calcium supplementation, drug-drug interactions, hypothyroidism, pharmacokinetics, serum thyroxine
- MeSH terms
- Absorption, Acetates, Adolescent, Adult, Calcium Carbonate, Calcium, Dietary, Drug Interactions, Drug Synergism, Female, Humans, Male, Thyroid Gland, Thyroxine, Calcium Compounds, Calcium Citrate
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