Thyroid Replacement Hormones (cont.)

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Are there any differences among the different types of thyroid replacement hormones?

There is conflicting evidence regarding which hormone replacement therapy should be preferred. The American Association of Clinical Endocrinologists recommends that clinical hypothyroidism is best treated with synthetic T4 levothyroxine (for example levothyroxine and sodium [Synthroid, Levoxyl and Levothroid]). There is variability between the absorption and distribution of generic T4 compared to brand name preparations. Hence it is recommended that patients remain with specific brand names during treatment. There is also variability between generic formulations and brand names of pure T3 (liothyronine [Cytomel, Triostat]), combined T4/T3 formulations (liotrix [Thyrolar]) and thyroid extracts from animal sources (Armour Thyroid, Nature-Throid etc.). Thyroid extracts from animal sources are no longer available in the United States. Emerging information shows that combination of T4/T3 therapy may have some advantages over T4 in cognitive performance and mood but studies are not conclusive.

With which drugs do thyroid replacement hormones interact?

Thyroid replacement hormones should be used cautiously in people with diabetes since starting or discontinuing therapy may lead to a loss of control of the blood sugar requiring adjustments in doses of insulin or oral antidiabetic drugs (for example, glyburide [Micronase]). The effects of blood thinners such as warfarin (Coumadin) may be increased by thyroid replacement hormones warranting a decrease in the dose of warfarin in addition to monitoring of blood clotting.

Intravenous epinephrine administration in patients with coronary artery disease who are taking thyroid replacement hormones may increase the risk of complications such as difficulty in breathing and possibly heart attacks. The effectiveness of some beta blockers [for example, metoprolol (Lopressor) orpropranolol (Inderal)] may be reduced when a patient is converted from a state of hypothyroidism (under activity) to a normal state (euthyroid state). It also may be necessary to modify the dose of digoxin (Lanoxin) and theophylline (Slo-Bid) when a patient is converted from hypothyroidism (under activity) to a normal state (euthyroid state). There is increased elimination of theophylline in a euthyroid state compared to a state of hypothyroidism.

Medically Reviewed by a Doctor on 3/27/2015

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