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Effects of grapefruit juice on the absorption of levothyroxine

2005; Wiley; Volume: 60; Issue: 3 Linguagem: Inglês

10.1111/j.1365-2125.2005.02433.x

1365-2125

Jari J. Lilja, Kalevi Laitinen, Pertti J. Neuvonen,

Amino Acid Enzymes and Metabolism

Aims Our aim was to study the effect of grapefruit juice on the pharmacokinetics of levothyroxine. Methods In a randomized cross‐over study with two phases, 10 healthy subjects ingested 200 ml grapefruit juice or water (control) three times daily for 2 days. On day 3, a single 600 µg dose of levothyroxine was administered with 200 ml grapefruit juice or water, which was also ingested 1 h before and 1 h after levothyroxine. Serum concentrations of total thyroxine (T4) and thyroid‐stimulating hormone (TSH) were measured up to 24 h. Results Grapefruit juice decreased slightly (11%; P < 0.01) the maximal increase of T4 concentration after ingestion of levothyroxine from 66.4 nmol l −1 to 59.4 nmol l −1 (95% CI on the difference −11.3, −2.7). The incremental areas under the serum T4 concentration‐time curve (dAUC) during the first 4 and 6 h were also decreased slightly: dAUC(0,4 h) by 13% ( P < 0.05), from 195 nmol l −1 h to 169 nmol l −1 h (95% CI −51, −1) and dAUC(0,6 h) by 9% ( P = 0.085), from 298 nmol l −1 h to 271 nmol l −1 h (95% CI −58, 4). The decrease in the serum concentration of TSH (1.25 mU l −1 ) measured 24 h after ingestion of levothyroxine, was not altered by grapefruit juice. Conclusions Grapefruit juice may slightly delay the absorption of levothyroxine, but it seems to have only a minor effect on its bioavailability. Accordingly, the clinical relevance of the grapefruit juice‐levothyroxine interaction is likely to be small.