General Information: Desiccated thyroid is a naturally occurring thyroid hormone derived from porcine thyroid glands. Thyroid hormone is used to treat hypothyroidism that results from primary atrophy of the gland, functional thyroid deficiency, or decreased thyroid function secondary to surgery, radiation, or antithyroid agents. The preparation is also used to treat pituitary hypothyroidism, hypothalamic hypothyroidism, and cretinism. Because of considerable variations in levothyroxine (T4) and liothyronine (T3) content, porcine thyroid hormone has been largely replaced in clinical medicine by synthetic levothyroxine, which has a more reliable hormonal content. Various thyroid extracts have received FDA approval since 1939.
Mechanism of Action: Desiccated thyroid contains both levothyroxine and liothyronine; these two hormones possess all the actions of endogenous thyroid hormones. Liothyronine (T3) is the principal hormone that exhibits these actions whereas levothyroxine (T4) is the major hormone secreted by the thyroid gland and is metabolically deiodinated to T3 in peripheral tissues. Serum concentrations of T4 and TSH are typically used as the primary monitoring parameters for determining thyroid function.
The actions of thyroid hormones are extensive. In general, thyroid hormones influence the growth and maturation of tissues, increase energy expenditure, and affect the turnover of essentially all substrates. These effects are mediated through control of DNA transcription and, ultimately, protein synthesis. Thyroid hormones play an integral role in both anabolic and catabolic processes and are particularly important to the development of the central nervous system in newborns. They regulate cell differentiation and proliferation, and aid in the myelination of nerves and the development of axonal and dendritic processes in the nervous system. Thyroid hormones, along with somatotropin, are responsible for regulating growth, particularly of bones and teeth. Thyroid hormones also decrease cholesterol concentrations in the liver and the bloodstream, and have a direct cardiostimulatory action. Cardiac consumption is increased by the administration of thyroid hormone, resulting in an increased cardiac output. Administration of exogenous thyroid hormone to patients with hypothyroidism increases the metabolic rate by enhancing protein and carbohydrate metabolism, increasing gluconeogenesis, facilitating the mobilization of glycogen stores, and increasing protein synthesis. In response to reestablishing physiologic levels of thyroid hormone, thyroid-stimulating hormone (TSH) concentrations correct if the primary disorder is at the level of the thyroid.
The release of T3 and T4 from the thyroid gland into the systemic circulation is regulated by TSH (thyrotropin), which is secreted by the anterior pituitary gland. Thyrotropin release is controlled by the secretion of thyroid-releasing hormone (TRH) from the hypothalamus and by a feedback mechanism dependent on the concentrations of circulating thyroid hormones. Because of this feedback mechanism, the administration of pharmacologic doses of exogenous thyroid hormone to patients with a normal thyroid suppresses endogenous thyroid hormone secretion.
Pregnancy: Thyroid hormones are considered FDA pregnancy risk category A drugs. Desiccated thyroid hormones undergo minimal placental transfer and human experience does not indicate adverse fetal effects; do not discontinue needed replacement during pregnancy.2Also, hypothyroidism diagnosed during pregnancy should be promptly treated. Measure TSH during each trimester to gauge adequacy of thyroid replacement dosage since during pregnancy thyroid requirements may increase. Immediately after obstetric delivery, dosage should return to the pre-pregnancy dose, monitor a serum TSH or other thyroid function tests 6—8 weeks postpartum to assess for needed adjustments.
Breast-feeding: Thyroid hormones, like desiccated thyroid, are generally compatible with breast-feeding; minimal amounts of thyroid hormones are excreted in breast milk.3 Thyroid hormones do not have a known tumorigenic potential and are not associated with serious adverse reactions in nursing infants. However, use caution when administering desiccated thyroid to a nursing woman 2; changes in thyroid status in the post-partum period may require careful monitoring and maternal dosage adjustment. It should be noted that in general, adequate thyroid status is needed to maintain normal lactation, and there is no reason maternal replacement should be halted due to lactation alone. Levothyroxine is often the preferential drug to treat hypothyroidism and is considered compatible with breast feeding.3
Interactions: Possible interactions include amiodarone; antacids; anti-thyroid medicines; calcium supplements; carbamazepine; cholestyramine; colestipol; digoxin; female hormones, including contraceptive or birth control pills; iron supplements; ketamine; liquid nutrition products; medicines for colds and breathing difficulties; medicines for diabetes; medicines for mental depression; barbiturates; phenytoin; corticosteroids; rifabutin; rifampin; soy isoflavones; sucralfate; theophylline; warfarin. This list may not describe all possible interactions. Give your health care provider a list of all the medicines, herbs, non-prescription drugs, or dietary supplements you use. Some items may interact with your medicine.