Drug Interactions between liothyronine and sodium iodide-i-131

MONITOR: Excessive intake of dietary iodine (e.g., iodised salt) may interfere with adequate uptake of radioiodide by the thyroid .

MANAGEMENT: A diet low in iodine is recommended before initiating therapy with sodium iodide I-131 or I-123.

ADJUST DOSING INTERVAL: Concurrent administration of calcium-containing products may decrease the oral bioavailability of levothyroxine by one-third in some patients. Pharmacologic effects of levothyroxine may be reduced. The exact mechanism of interaction is unknown but may involve nonspecific adsorption of levothyroxine to calcium at acidic pH levels, resulting in an insoluble complex that is poorly absorbed from the gastrointestinal tract. In one study, 20 patients with hypothyroidism who were taking a stable long-term regimen of levothyroxine demonstrated modest but significant decreases in mean free and total thyroxine (T4) levels as well as a corresponding increase in mean thyrotropin (thyroid-stimulating hormone, or TSH) level following the addition of calcium carbonate (1200 mg/day of elemental calcium) for 3 months. Four patients had serum TSH levels that were higher than the normal range. Both T4 and TSH levels returned to near-baseline 2 months after discontinuation of calcium, which further supported the likelihood of an interaction. In addition, there have been case reports suggesting decreased efficacy of levothyroxine during calcium coadministration. It is not known whether this interaction occurs with other thyroid hormone preparations.

MANAGEMENT: Some experts recommend separating the times of administration of levothyroxine and calcium-containing preparations by at least 4 hours. Monitoring of serum TSH levels is recommended. Patients with gastrointestinal or malabsorption disorders may be at a greater risk of developing clinical or subclinical hypothyroidism due to this interaction.

ADJUST DOSING INTERVAL: Certain drugs may affect the pharmacokinetics, pharmacodynamics, and/or diagnostic results of radioiodides.

MANAGEMENT: Antithyroid agents such as carbimazole or propylthiouracil should generally be withheld for 1 week before administration of sodium iodide I-131 or I-123, and for a few days afterward. Salicylates, steroids, anticoagulants, antihistamines, antiparasitics, penicillins, sulfonamides, tolbutamide, thiopental, and nitroprusside should generally be withheld for 1 week. Amiodarone, lithium, and benzodiazepines should be withheld for at least 4 weeks.