Drug Interaction Report
3 potential interactions and/or warnings found for the following 2 drugs:
- Cytomel (liothyronine)
- Ferro Basic (docusate / ferrous fumarate)
Interactions between your drugs
liothyronine ferrous fumarate
Applies to: Cytomel (liothyronine), Ferro Basic (docusate / ferrous fumarate)
ADJUST DOSING INTERVAL: Concurrent administration of iron-containing products may decrease the oral bioavailability and pharmacologic effects of levothyroxine. The exact mechanism of interaction is unknown, but in vitro data suggest it may involve nonspecific adsorption of levothyroxine to iron at acidic pH levels, resulting in an insoluble complex that is poorly absorbed from the gastrointestinal tract. In one study, 14 patients with hypothyroidism who were taking a stable long-term regimen of levothyroxine demonstrated an increase in mean serum thyrotropin (thyroid-stimulating hormone, or TSH) level from 1.6 to 5.4 mU/L following the addition of ferrous sulfate (300 mg administered simultaneously with levothyroxine 30 to 60 minutes before breakfast) for 3 months. A total of 11 patients had increases in serum TSH at week 12 compared to baseline, including two that had levels above the upper limit of normal for the assay, indicating the presence of hypothyroidism. Nine of the eleven also had an increase in signs and symptoms of hypothyroidism based on subjective evaluation using a clinical score. It is not known whether this interaction occurs with other thyroid hormone preparations.
MANAGEMENT: Some authorities recommend separating the times of administration of levothyroxine and iron-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.
References
- Campbell NR, Hasinoff BB, Stalts H, Rao B, Wong NC (1992) "Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism." Ann Intern Med, 117, p. 1010-3
- (2024) "Product Information. Levothyroxine Sodium (levothyroxine)." Lannett Company Inc
- (2023) "Product Information. Levothyroxine Sodium (levothyroxine)." Zentiva Pharma UK Ltd
- (2023) "Product Information. Levothyroxine (Sandoz) (levothyroxine sodium)." Sandoz Pty Ltd
Drug and food interactions
ferrous fumarate food
Applies to: Ferro Basic (docusate / ferrous fumarate)
ADJUST DOSING INTERVAL: Concomitant use of some oral medications may reduce the bioavailability of orally administered iron, and vice versa.
Food taken in conjunction with oral iron supplements may reduce the bioavailability of the iron. However, in many patients intolerable gastrointestinal side effects occur necessitating administration with food.
MANAGEMENT: Ideally, iron products should be taken on an empty stomach (i.e., at least 1 hour before or 2 hours after meals), but if this is not possible, administer with meals and monitor the patient more closely for a subtherapeutic effect. Some studies suggest administration of iron with ascorbic acid may enhance bioavailability. In addition, administration of oral iron products and some oral medications should be separated whenever the bioavailability of either agent may be decreased. Consult the product labeling for specific separation times and monitor clinical responses as appropriate.
References
- "Product Information. Feosol (ferrous sulfate)." SmithKline Beecham
- (2021) "Product Information. Accrufer (ferric maltol)." Shield Therapeutics
liothyronine food
Applies to: Cytomel (liothyronine)
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.
References
- Schneyer CR (1998) "Calcium carbonate and reduction of levothyroxine efficacy." JAMA, 279, p. 750
- Singh N, Singh PN, Hershman JM (2000) "Effect of calcium carbonate on the absorption of levothyroxine." JAMA, 283, p. 2822-5
- Csako G, McGriff NJ, Rotman-Pikielny P, Sarlis NJ, Pucino F (2001) "Exaggerated levothyroxine malabsorption due to calcium carbonate supplementation in gastrointestinal disorders." Ann Pharmacother, 35, p. 1578-83
- Neafsey PJ (2004) "Levothyroxine and calcium interaction: timing is everything." Home Healthc Nurse, 22, p. 338-9
Therapeutic duplication warnings
No duplication warnings were found for your selected drugs.
Therapeutic duplication warnings are only returned when drugs within the same group exceed the recommended therapeutic duplication maximum.
Drug Interaction Classification
Highly clinically significant. Avoid combinations; the risk of the interaction outweighs the benefit. | |
Moderately clinically significant. Usually avoid combinations; use it only under special circumstances. | |
Minimally clinically significant. Minimize risk; assess risk and consider an alternative drug, take steps to circumvent the interaction risk and/or institute a monitoring plan. | |
No interaction information available. |
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