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Drug Interaction Report

3 potential interactions and/or warnings found for the following 2 drugs:

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Interactions between your drugs

Moderate

levothyroxine testosterone

Applies to: Levotec (levothyroxine), Meditest (testosterone)

MONITOR: Androgens may induce reversible clinical hyperthyroidism in patients receiving thyroid hormone replacement therapy. The proposed mechanism is androgen-induced decrease in T4 binding globulin resulting in decreased serum T4, increased T3 uptake resin and free T4, and decreased TSH levels.

MANAGEMENT: Clinical and laboratory monitoring of thyroid function may be necessary, as may a 25% to 50% reduction in thyroid hormone dosage.

References

  1. Arafah BM (1994) "Decreased levothyroxine requirement in women with hypothyroidism during androgen therapy for breast cancer." Ann Intern Med, 121, p. 247-51
  2. (2024) "Product Information. Synthroid (levothyroxine)." AbbVie US LLC
  3. (2024) "Product Information. Levothyroxine Sodium (levothyroxine)." Lannett Company Inc
  4. (2023) "Product Information. Levothyroxine Sodium (levothyroxine)." Zentiva Pharma UK Ltd
  5. (2023) "Product Information. Levothyroxine (Sandoz) (levothyroxine sodium)." Sandoz Pty Ltd
  6. (2024) "Product Information. Liothyronine Sodium (liothyronine)." AvKare Inc
  7. (2024) "Product Information. Eltroxin (levothyroxine)." Shire US Inc
  8. (2024) "Product Information. Armour Thyroid (thyroid desiccated)." A-S Medication Solutions
View all 8 references

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Drug and food interactions

Moderate

levothyroxine food

Applies to: Levotec (levothyroxine)

ADJUST DOSING INTERVAL: Consumption of certain foods as well as the timing of meals relative to dosing may affect the oral absorption of T4 thyroid hormone (i.e., levothyroxine). T4 oral absorption is increased by fasting and decreased by foods such as soybean flour (e.g., infant formula), cotton seed meal, walnuts, dietary fiber, calcium, and calcium fortified juices. Grapefruit or grapefruit products may delay the absorption of T4 thyroid hormone and reduce its bioavailability. The mechanism of this interaction is not fully understood.

MANAGEMENT: Some manufacturers recommend administering oral T4 as a single daily dose, on an empty stomach, one-half to one hour before breakfast. In general, oral preparations containing T4 thyroid hormone should be administered on a consistent schedule with regard to time of day and relation to meals to avoid large fluctuations in serum levels. Foods that may affect T4 absorption should be avoided within several hours of dosing if possible. Consult local guidelines for the administration of T4 in patients receiving enteral feeding.

References

  1. (2002) "Product Information. Synthroid (levothyroxine)." Abbott Pharmaceutical
  2. (2022) "Product Information. Armour Thyroid (thyroid desiccated)." Forest Pharmaceuticals
  3. Wohlt PD, Zheng L, Gunderson S, Balzar SA, Johnson BD, Fish JT (2009) "Recommendations for the use of medications with continuous enteral nutrition." Am J Health Syst Pharm, 66, p. 1438-67

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Moderate

levothyroxine food

Applies to: Levotec (levothyroxine)

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

  1. Schneyer CR (1998) "Calcium carbonate and reduction of levothyroxine efficacy." JAMA, 279, p. 750
  2. Singh N, Singh PN, Hershman JM (2000) "Effect of calcium carbonate on the absorption of levothyroxine." JAMA, 283, p. 2822-5
  3. 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
  4. Neafsey PJ (2004) "Levothyroxine and calcium interaction: timing is everything." Home Healthc Nurse, 22, p. 338-9
View all 4 references

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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.


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Drug Interaction Classification

These classifications are only a guideline. The relevance of a particular drug interaction to a specific individual is difficult to determine. Always consult your healthcare provider before starting or stopping any medication.
Major Highly clinically significant. Avoid combinations; the risk of the interaction outweighs the benefit.
Moderate Moderately clinically significant. Usually avoid combinations; use it only under special circumstances.
Minor 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.
Unknown No interaction information available.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.