indication

May be used to treat primary, secondary or tertiary hypothyroidism. May also be used to suppress thyroid stimulating hormone (TSH) secretion in patients with simple (nontoxic) goiter, subacute or chronic lymphocytic thyroiditis multinodular goiter, and in the management of thyroid cancer. May be used in conjunction with other antithyroid agents to treat thyrotoxicosis to prevent goitrogenesis and hypothyroidism. May also be used for differential diagnosis of suspected mild hyperthyroidism or thyroid gland autonomy.

pharmacology

Thyroid hormone drugs are natural or synthetic preparations containing T₄ or T₃ or both. T₄ and T₃ are produced in the human thyroid gland by the iodination and coupling of the amino acid tyrosine. Liotrix is a synthetic preparation of T4 and T3 in a 4:1 weight-based ratio. These hormones enhance oxygen consumption by most tissues of the body and increase the basal metabolic rate and the metabolism of carbohydrates, lipids and proteins. Thus, they exert a profound influence on every organ system in the body and are of particular importance in the development of the central nervous system.

mechanism of action

The hormones, T₄ and T₃, are tyrosine-based hormones produced by the thyroid gland. Iodine is an important component in their synthesis. The major secreted form of thyroid hormone is T4. T4 is converted T3, the more active thyroid hormone, by deiodinases in peripheral tissues. T3 acts in the body to increase basal metabolic rate, alter protein synthesis and increase the body's sensitivity to catecholamines (such as adrenaline). Thyroid hormones are essential for proper development and differentiation of all cells of the human body. T₄ and T₃ regulate protein, fat and carbohydrate metabolism to varying extents. The most pronounced effect of the hormones is in altering how human cells use energetic compounds. The thyroid hormone derivatives bind to the thyroid hormone receptors initially to initiate their downstream effects.

toxicity

Hypermetabolic state indistinguishable from thyrotoxicosis of endogenous origin. Symptoms of thyrotoxicosis include weight loss, increased appetite, palpitations, nervousness, diarrhea, abdominal cramps, sweating, tachycardia, increased pulse and blood pressure, cardiac arrhythmias, tremors, insomnia, heat intolerance, fever, and menstrual irregularities.

route of elimination

Thyroid hormones are primarily eliminated by the kidneys.

drug interactions

Calcium Acetate: Calcium salts such as calcium acetate may diminish the therapeutic effect of thyroid products such as liotrix. Separate the doses of the thyroid product and the oral calcium supplement by at least 4 hours. Monitor for decreased therapeutic effects of thyroid products if an oral calcium supplement is initiated/dose increased, or increased effects if an oral calcium supplement is discontinued/dose decreased.

Cholestyramine: The resin, cholestyramine, decreases the absorption of the thyroid hormone, liotrix.

Colestipol: The resin, colestipol, decreases the absorption of the thyroid hormone, liotrix.

Digoxin: The thyroid hormone, liotrix, decreases the effect of digoxin.

Warfarin: Liotrix may contribute to the anticoagulant effect of warfarin by increasing the metabolism of vitamin K-dependent clotting factors. Monitor for changes in prothrombin time and anticoagulant effects if liotrix is initiated, discontinued or dose changed.