McKenna's Pharmacology for Nursing, 2e

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P A R T 6  Drugs acting on the endocrine system

A ntithyroid agents Drugs used to block the production of thyroid hormone and to treat hyperthyroidism include the thioamides and iodide solutions (Table 37.2). Although these groups of drugs are not chemically related, they both block the formation of thyroid hormones within the thyroid gland (see Therapeutic actions and indications). Thioamides lower thyroid hormone levels by prevent- ing the formation of thyroid hormone in the thyroid cells, which lowers the serum levels of thyroid hormone. They also partially inhibit the conversion of T 4 to T 3 at the cellular level. These drugs are indicated for the treatment of hyperthyroidism. Thioamides include carbimazole ( Neo-Mercazole ) and propylthiouracil (PTU). Carbimazole is the most commonly used drug in New Zealand. PTU is an unapproved medicine in New Zealand reserved for people who are intolerant to carbimazole. Iodine solutions Low doses of iodine are needed in the body for the for- mation of thyroid hormone. High doses, however, block thyroid function. Therefore, iodine preparations are sometimes used to treat hyperthyroidism but are not used as often as they once were in the clinical setting (see Pharmacokinetics). The iodine solutions cause the thyroid cells to become oversaturated with iodine and stop producing thyroid hormone. In some cases, the thyroid cells are actually destroyed. Radioactive iodine (sodium iodide 131 I) (not available in New Zealand) is taken up into the thyroid cells, which are then destroyed by the beta-radiation given off by the radioactive iodine. Except during radiation emergencies, the use of sodium iodide is reserved for individuals who are not candidates for surgery, women who cannot become pregnant and elderly people with such severe, complicating conditions that immediate thyroid destruction is needed. Iodine solutions include strong iodine solution, potassium iodide and sodium iodide 131 I (generic). See Table 37.2 for usual indications for each drug. This would alert any healthcare personnel taking care of you in an emergency to the fact that you are taking this drug. • While you are taking this drug, you will need regular medical follow-up, including blood tests to check the activity of your thyroid gland, to evaluate your response to the drug and any possible underlying problems. Therapeutic actions and indications Thioamides

• Keep this drug, and all medications, out of the reach of children. Do not give this medication to anyone else or take any similar medication that has not been prescribed for you.

Pharmacokinetics Thioamides

These drugs are well absorbed from the GI tract and are then concentrated in the thyroid gland. The onset and duration varies with each person. PTU has a low poten- tial for crossing the placenta and for entering breast milk (see Contraindications and cautions). Iodine solutions These drugs are rapidly absorbed from the GI tract and widely distributed throughout the body fluids. Excretion occurs through the urine. Strong iodine products, potas- sium iodide and sodium iodide are taken orally and have a rapid onset of action, with effects seen within 24 hours and peak effects seen in 10 to 15 days. The effects are short lived and may even precipitate further thyroid enlargement and dysfunction (see Adverse effects). For this reason, the drugs are not used as often as they once were in the clinical setting. The strong iodine products cross the placenta and are known to enter breast milk, but the effects on the neonate are not known. Sodium iodide 131 I enters breast milk and is rated pregnancy category X (see Contra­ indications and cautions). Contraindications and cautions Antithyroid agents are contraindicated in the presence of any known allergy to antithyroid drugs to prevent hypersensitivity reactions and during pregnancy because of the risk of adverse effects on the fetus and the development of cretinism . (If an antithyroid drug is absolutely essential and the woman has been informed about the risk of cretinism in the infant, PTU is the drug of choice, but caution should still be used.) Another

Safe medication administration

Name confusion has been reported between propylthiouracil (PTU) and Puri-Nethol (mercaptopurine), an antineoplastic agent. Serious adverse effects could occur. Use extreme caution when using these drugs.