The Acute Effect of Calcium Carbonate on the Intestinal Absorption of Levothyroxine

To determine the acute effect of calcium, we measured levothyroxine absorption after ingestion of thyroxine (T4) with and without simultaneous ingestion of calcium (as calcium carbonate) in seven volunteers without thyroid disease. Serum total T₄, total triiodothyronine (T₃), free T₄, and thyrotropin (TSH) levels were measured after ingestion of 1,000 μg of levothyroxine on two separate visits at 4-week intervals: (1) levothyroxine alone and (2) levothyroxine together with 2.0 g of calcium as calcium carbonate. The amount of absorbed levothyroxine was calculated as the incremental rise in serum T₄ level during the first 6 hours multiplied by the volume of distribution for the hormone. When 1,000 μg of levothyroxine alone was given to subjects, the maximum average total T₄ absorption was 837 μg (83.7% of the dose ingested) at 120 minutes. When levothyroxine was coadministered with 2.0 g of calcium (as calcium carbonate), the maximum average T₄ absorption decreased to 579 μg (57.9% of the dose ingested) at 240 minutes. The total levothyroxine absorption over 6 hours was significantly greater with thyroxine than that with thyroxine and calcium (p = 0.02). The administration of calcium and levothyroxine in these subjects was associated with a significant reduction in the peak increment in serum total T₄(p = 0.02) and free T₄ levels (p = 0.03), as well as a significant reduction in the overall increment in serum total T₄ (p = 0.003), free T₄ (p= 0.002), and total T₃ levels (p = 0.01) over four time points (120 minutes, 240 minutes, 360 minutes, 1,440 minutes). In summary, this pharmacokinetic study in seven volunteers indicates that calcium carbonate acutely reduces T₄ absorption.