Thyroid Agents: Generic and Brand Names
- Thyroid hormones
- levothyroxine (Synthroid, Levoxyl)
- liothyronine (Cytomel, Triostat)
- liotrix (Thyrolar)
- thyroid desiccated extract (Armour Thyroid)
- Antithyroid agents
- Thioamides
- methimazole (Tapazole)
- propylthiouracil (PTU)
- iodine solutions (Sodium iodide I131)
- strong iodine solution, potassium iodide (Thyro-block)
- Thioamides
Disease Spotlight: Thyroid Dysfunction
Thyroid dysfunction is either underactivity (hypothyroidism) or overactivity (hyperthyroidism), at any age. Hypothyroidism is too little thyroid hormone to maintain normal metabolism, the most common type, common in older women and men, with obesity and fatigue among the symptoms. Severe adult hypothyroidism is called myxedema. Cretinism is the condition in children born without a thyroid gland or with a nonfunctioning one, leading to poor growth and development and intellectual disability. Hyperthyroidism is excessive thyroid hormone in circulation, most often from Graves' disease.
Thyroid Hormones
Thyroid hormones replace low or absent natural hormone and suppress pituitary overproduction of TSH. They come in natural and synthetic forms.
Therapeutic Action
As replacement for hypothyroid states (myxedema coma, goiters, thyroid cancer), thyroid hormone raises the metabolic rate of body tissues, increasing oxygen consumption, respiration, heart rate, growth and maturation, and the metabolism of fats, carbohydrates, and proteins. Levothyroxine, a synthetic salt of T4, is the most frequently used replacement because of its predictable bioavailability and reliability.
Indications
Replacement for hypothyroid states (myxedema coma, goiters, thyroid cancer); treatment of thyroid toxicity in conjunction with antithyroid drugs; and treatment of thyroid overstimulation during pregnancy.
Pharmacokinetics
Here are the characteristic interactions of thyroid hormones and the body in terms of absorption, distribution, metabolism, and excretion:
| Route | Onset | Peak | Duration |
|---|---|---|---|
| PO | Slow | 1-3 wk | 1-3 wk |
| IV | 6-8 h | 24-48 h | Unknown |
| Half-life (T1/2) | Metabolism | Excretion |
|---|---|---|
| 6-7 d | Liver | Bile |
Contraindications and Cautions
Allergy to any drug component. Acute thyrotoxicosis, unless used with antithyroid drugs. Myocardial infarction, which the drug can worsen. Lactation, since the drug enters breast milk and can suppress the infant's thyroid production. Addison disease, where the body cannot handle the drug effects. Pregnancy. Liothyronine and liotrix carry a greater incidence of cardiac side effects.
Adverse Effects
Skin reactions and hair loss, and symptoms of hyperthyroidism.
Interactions
Cholestyramine decreases absorption of thyroid hormones, so take them two hours apart. Anticoagulants increase bleeding. Thyroid hormones decrease the effectiveness of digitalis glycosides.
Nursing Considerations
Screen for allergy, pregnancy, and Addison disease, and get a baseline (skin lesions; orientation and affect; blood pressure, pulse, peripheral perfusion, and vessel evaluation; respirations and adventitious sounds; thyroid function tests). Give a single daily dose on an empty stomach, one-half to one hour before breakfast, with a full glass of water to keep levels consistent and prevent difficulty swallowing and esophageal irritation (FDA levothyroxine labeling). Monitor cardiac response and arrange periodic thyroid function tests. Provide comfort measures (temperature control, rest, safety precautions). Teach drug effects and the warning signs to report. Evaluate against return of metabolism to normal and prevention of goiter, and watch for tachycardia, hypertension, anxiety, and skin rash.
Antithyroid Agents
Antithyroid agents block thyroid hormone production to treat hyperthyroidism. They fall into two chemically unrelated groups, thioamides and iodide solutions, both of which block hormone formation within the thyroid gland.
Therapeutic Action
Thioamides lower thyroid hormone by preventing its formation in the thyroid cells and partially inhibiting the conversion of T4 to T3 at the cellular level; they include propylthiouracil (PTU) and methimazole. Iodine solutions in high doses block thyroid function by oversaturating the cells with iodine so they stop producing hormone.
Indications
Treatment of hyperthyroidism, and thyroid blocking in a radiation emergency.
Pharmacokinetics
Thioamides:
- Route: PO
- Onset: 30-60 min
- Peak: Not specified
- Duration: 2-4 h
- Half-life (T1/2): 6-13 h
- Metabolism: Not specified
Iodine solutions:
- Route: PO
- Onset: 24 h
- Peak: 10-15 days
- Duration: 6 weeks
- Half-life (T1/2): Unknown
- Metabolism: Liver
- Excretion: Urine
Contraindications and Cautions
Allergy to any drug component. Pregnancy (risk of cretinism); PTU is the drug of choice for pregnant women. Lactation, for the risk of antithyroid activity in the infant (neonatal goiter). Pulmonary edema or tuberculosis contraindicate strong iodine products.
Adverse Effects
Thioamides: drowsiness, lethargy, bradycardia, nausea, skin rash. PTU: nausea, vomiting, GI complaints, severe liver toxicity. Methimazole: bone marrow suppression. Iodine solution: hypothyroidism; metallic taste and burning in the mouth, sore teeth and gums, diarrhea; staining of teeth, skin rash, and goiter. I131 is only for patients over 30 years old because of the risks of radioactivity.
Interactions
Thioamides: increased bleeding with oral anticoagulants and PTU. Iodine solutions: changes in the metabolism and level of anticoagulants, theophylline, digoxin, metoprolol, and propranolol.
Nursing Considerations
Screen for allergy, renal stone, and pregnancy, and get a baseline (skin lesions; orientation and affect; liver evaluation; serum calcium, magnesium, and alkaline phosphatase levels; and bone radiographs as appropriate). Give PTU three times a day, around the clock, for consistent levels. Give iodine solution through a straw to reduce teeth staining; tablets can be crushed. Provide comfort measures. Teach drug effects and the warning signs to report. Evaluate against lowering of thyroid hormone levels, and watch for bradycardia, anxiety, and blood dyscrasias. Methimazole carries a risk of agranulocytosis, so teach patients to report fever or sore throat right away (FDA methimazole labeling).
Frequently Asked Questions
What is the difference between thyroid hormones and antithyroid agents? Thyroid hormones (such as levothyroxine) replace hormone the gland cannot make and treat hypothyroidism. Antithyroid agents (thioamides and iodine solutions) block hormone production and treat hyperthyroidism.
Why is levothyroxine the preferred thyroid replacement? Levothyroxine is a synthetic salt of T4 with predictable bioavailability and reliability, which makes dosing consistent. Per FDA labeling it is taken once daily on an empty stomach, one-half to one hour before breakfast, and peak therapeutic effect is not reached for several weeks.
Which antithyroid drug is preferred in pregnancy? Propylthiouracil (PTU) is the drug of choice in pregnant patients. Thioamides cross the placenta and carry a risk of fetal goiter and cretinism, so the lowest effective dose is used.
What is the most serious side effect of methimazole? Agranulocytosis. FDA labeling directs patients to report any fever or sore throat promptly, since these can signal a dangerous drop in white blood cells.
How should a nurse give thyroid hormone replacement? Give a single daily dose on an empty stomach before breakfast with a full glass of water, separate it from cholestyramine by about two hours, monitor cardiac response, and arrange periodic thyroid function tests.
Why must levothyroxine never be used for weight loss? FDA labeling warns that thyroid hormones are not effective for obesity, and larger doses can cause serious or life-threatening toxicity, especially when combined with sympathomimetic amines.