Effect of Calcium Carbonate on the Absorption of Levothyroxine

TLDR: This study of 20 patients receiving long-term levothyroxine replacement therapy indicates that calcium carbonate reduces T(4) absorption and increases serum thyrotropin levels.

Abstract: ContextThe effect of calcium carbonate on the absorption of levothyroxine has not been studied systematically. Such a potential drug interaction merits investigation because concurrent treatment with both drugs is common, particularly in postmenopausal women.ObjectiveTo investigate the potential interference of calcium carbonate in the absorption of levothyroxine.DesignProspective cohort study conducted from November 1998 to June 1999, supplemented with an in vitro study of thyroxine (T4) binding to calcium carbonate.SettingVeterans Affairs Medical Center in West Los Angeles, Calif.PatientsTwenty patients (age range, 27-78 years; n=11 men) with hypothyroidism who were taking a stable long-term regimen of levothyroxine were included in the study. All patients had serum free T4 and thyrotropin values in the normal range before beginning the study.InterventionSubjects were instructed to take 1200 mg/d of elemental calcium as calcium carbonate, ingested with their levothyroxine, for 3 months.Main Outcome MeasuresLevels of free T4, total T4, total triiodothyronine (T3), and thyrotropin, measured in all subjects at baseline (while taking levothyroxine alone), at 2 and 3 months (while taking calcium carbonate and levothyroxine), and 2 months after calcium carbonate discontinuation (while continuing to take levothyroxine).ResultsMean free T4 and total T4 levels were significantly reduced during the calcium period and increased after calcium discontinuation. Mean free T4 levels were 17 pmol/L (1.3 ng/dL) at baseline, 15 pmol/L (1.2 ng/dL) during the calcium period, and 18 pmol/L (1.4 ng/dL) after calcium discontinuation (overall P<.001); mean total T4 levels were 118 nmol/L (9.2 µg/dL) at baseline, 111 nmol/L (8.6 µg/dL) during the calcium period, and 120 nmol/L (9.3 µg/dL) after calcium discontinuation (overall P=.03). Mean thyrotropin levels increased significantly, from 1.6 mIU/L at baseline to 2.7 mIU/L during the calcium period, and decreased to 1.4 mIU/L after calcium discontinuation (P=.008). Twenty percent of patients had serum thyrotropin levels higher than the normal range during the calcium period; the highest observed level was 7.8 mIU/L. Mean T3 levels did not change during the calcium period. The in vitro study of T4 binding to calcium showed that adsorption of T4 to calcium carbonate occurs at acidic pH levels.ConclusionsThis study of 20 patients receiving long-term levothyroxine replacement therapy indicates that calcium carbonate reduces T4 absorption and increases serum thyrotropin levels. Levothyroxine adsorbs to calcium carbonate in an acidic environment, which may reduce its bioavailability.