THYROID PREPARATIONS

• ACRONYMS AND DEFINITIONS

• Levothyroxine (L-thyroxine) - pharmaceutical form of T4. Levothyroxine is identical to thyroxine secreted by the thyroid gland.
• Liothyronine (L-triiodothyronine) - pharmaceutical form of T3
• T3 (triiodothyronine) - physiologically active form of thyroid hormone. T4 is converted to T3 in the periphery.
• T4 (thyroxine) - main form of thyroid hormone secreted by the thyroid gland

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• LEVOTHYROXINE (T4)

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• LIOTHYRONINE (T3)

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• COMBINATION PRODUCTS (T4 + T3)

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• Food interactions
• Certain foods decreases the absorption of thyroid hormone
• Thyroid hormone should be taken on an empty stomach, preferably 30 - 60 minutes before breakfast or ≥ 3 hours after the evening meal


• Foods that are known to decrease the absorption of thyroid hormone
• Soybean flour (infant formula)
• Cottonseed meal
• Walnuts
• Dietary fiber
• Grapefruit juice

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• DRUG INTERACTIONS

• NOTE: Drug interactions presented here are NOT all-inclusive. Other interactions may exist. The interactions presented here are meant to encompass commonly prescribed medications and/or interactions that are well-documented. Always consult your physician or pharmacist before taking medications concurrently. CLICK HERE for more information on drug interactions.


• Drugs that decrease the absorption of thyroid hormone
• Bile acid sequestrants (e.g. cholestyramine, Welchol) - take thyroid hormone at least 4 hours before these drugs
• Calcium carbonate - do not take within 4 hours of each other
• Drugs that raise gastric pH (PPIs, antacids, H2 antagonists, sucralfate) - may decrease absorption of thyroid hormone
• Ferrous sulfate - do not take within 4 hours of each other
• Ion exchange resins (kayexalate, sevelamer) - take thyroid hormone at least 4 hours before these drugs
• Orlistat (Xenical®) - may decrease absorption


• Drugs that increase thyroid hormone metabolism
• Phenobarbital - Phenobarbital has been shown to reduce the response to thyroxine. Phenobarbital increases L-thyroxine metabolism by inducing uridine 5'-diphospho-glucuronosyltransferase (UGT) and leads to a lower T4 serum levels. Changes in thyroid status may occur if barbiturates are added or withdrawn from patients being treated for hypothyroidism.
• Rifampin - rifampin has been shown to accelerate the metabolism of levothyroxine


• Drugs that may decrease the peripheral conversion of T4 to T3
• Beta blockers - in patients treated with large doses of propranolol (> 160 mg/day), T3 and T4 levels change, TSH levels remain normal, and patients are clinically euthyroid
• Glucocorticoids - short-term administration of large doses of glucocorticoids may decrease serum T3 concentrations by 30% with minimal change in serum T4 levels. However, long-term glucocorticoid therapy may result in slightly decreased T3 and T4 levels due to decreased TBG production.
• Amiodarone - amiodarone inhibits peripheral conversion of levothyroxine (T4) to triiodothyronine (T3) and may cause isolated biochemical changes (increase in serum free T4, and decrease or normal free T3) in clinically euthyroid patients


• Drugs that may cause an initial transient increase in free T4, but continued administration results in a decrease in serum T4 and normal free T4 and TSH concentrations
• Salicylates (> 2 g/day) - Salicylates inhibit binding of T4 and T3 to TBG and transthyretin. An initial increase in serum free T4 is followed by return of free T4 to normal levels with sustained therapeutic serum salicylate concentrations, although total T4 levels may decrease by as much as 30%.
• Furosemide (> 80 mg IV) - Furosemide has been shown to inhibit the protein binding of T4 to TBG and albumin, causing an increased free T4 fraction in serum. Furosemide competes for T4-binding sites on TBG, prealbumin, and albumin, so that a single high dose can acutely lower the total T4 level
• Phenytoin (Dilantin®) - phenytoin reduces serum protein binding of levothyroxine, and total and free T4 may be reduced by 20% to 40%, but most patients have normal serum TSH levels and are clinically euthyroid.
• Carbamazepine (Tegretol®) - carbamazepine reduces serum protein binding of levothyroxine, and total and free T4 may be reduced by 20% to 40%, but most patients have normal serum TSH levels and are clinically euthyroid.
• NSAIDs (mefenamic acid, meclofenamate)
• Heparin


• Drugs that may increase serum thyroxine-binding globulin (TBG) without affecting free T4 concentrations
• Clofibrate
• Estrogen-containing oral contraceptives
• Estrogens (oral)
• Heroin / Methadone
• 5-Fluorouracil
• Mitotane
• Tamoxifen


• Drugs that may decrease serum thyroxine-binding globulin (TBG) without affecting free T4 concentrations
• Androgens / Anabolic Steroids
• Asparaginase
• Estrogens (oral)
• Glucocorticoids
• Slow-Release Nicotinic Acid


• Other potential drug interactions
• Diabetes medications - thyroid hormone may worsen diabetic control
• Anticoagulants - thyroid hormone may increase the response to anticoagulants
• Digoxin - thyroid hormone may decrease the response to digoxin. Thyroid hormone may decrease digoxin levels.
• Tricyclic antidepressants - thyroid hormone may potentiate the effects of tricyclic antidepressants
• Sertraline (Zoloft®) - patients on sertraline may have increased thyroid hormone requirements
• Ketamine - concurrent thyroid hormone and ketamine may produce marked hypertension and tachycardia
• Sympathomimetics - concurrent thyroid hormone and sympathomimetics may increase the effects of sympathomimetics or thyroid hormone
• Tyrosine-kinase inhibitors - concurrent use of tyrosine-kinase inhibitors such as imatinib may cause hypothyroidism. Closely monitor TSH levels in such patients.

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• PRICE ($) INFO

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• BIBLIOGRAPHY

• Manufacturer's package insert