An Overview of Thyroid and Antithyroid Drugs
About the Thyroid
The thyroid gland is the largest endocrine gland in the body. It produces three hormones: thyroxine (T4), triiodothyronine (T3), and calcitonin, which functions in calcium metabolism. The hypothalamus secretes thyrotropin measuring hormone, or TRH, which causes the pituitary gland to produce thyroid-stimulating hormone, or TSH, which stimulates the thyroid gland to produce thyroid hormones.
Thyroid hormones control the rate of cellular metabolism, therefore influencing the functioning of nearly every cell in the body. The heart, liver, kidneys, and skeletal muscle are especially responsive to the stimulating effects of thyroid hormones. Thyroid hormones are required for normal growth and development and are considered especially critical for brain and skeletal development and maturation.
Hypothyroidism and hyperthyroidism are two types of thyroid disorders requiring drug therapy. These disorders produce opposing effects on the body. For more details on hyperthyroidism, see our video on symptoms of hyperthyroidism. Thyroid hormone is required for normal body functioning and for life.
Thyroid and Antithyroid Drugs and Their Effects
The goal of treatment with thyroid and antithyroid drugs is to restore the patient to a euthyroid state (having a normal thyroid function with normal metabolism). Thyroid drugs provide an exogenous source of thyroid hormone. They’re indicated for hypothyroidism, cretinism (which is a childhood congenital form), and myxedema (a severe adult form).
Antithyroid drugs act by decreasing production or release of thyroid hormones. They are given for hyperthyroidism associated with Graves’ disease, nodular goiter, thyroiditis, overtreatment with thyroid drugs, thyroid carcinoma, pituitary adenoma that secretes excessive amounts of TSH, and thyroid storm. For more details on Graves’ disease and thyroid storm, check out our Graves’ disease video.
There is a black box warning from the FDA concerning the use of thyroid hormones for the treatment of weight loss. There is no evidence that thyroid preparations lead to weight loss, and large doses are associated with life-threatening toxicity. The goal of thyroid replacement therapy and hypothyroidism is to administer a sufficient dosage to compensate for the thyroid deficit, so as to restore normal concentrations of thyroid hormones and provide relief of symptoms.
Levothyroxine, also known as synthroid, levoxyl, and levothroid, is a synthetic form of thyroxine (T4) and is the standard replacement therapy for patients with hyperthyroidism. The medication is given orally and is best absorbed on an empty stomach. It is, therefore, recommended to give in the morning 30 minutes before breakfast. It should also be taken alone, because other medications can decrease its effect. Check the pulse rate before giving. A rate over 100 may indicate adverse cardiac effects. It takes approximately 4-6 weeks to achieve a steady state after initiation of the drug. For most patients, the daily dose ranges from 50-200 micrograms per day. Different formulations at the same dose can result in different TSH values in an individual. Therefore, whether brand name or generic, it is important for the patient to stick with one formulation. For congenital hyperthyroidism, or cretinism, drug therapy should be started within six weeks of birth and continued for life or mental retardation may result.
Therapeutic effects of thyroid drugs include returning to normal metabolic activities, including increased energy and activity level, increased alertness and appetite, increased pulse rate and temperature, decreased constipation, reversal of skin and hair problems, decreased serum cholesterol.
Adverse effects of thyroid drugs usually stem from excessive doses and are the signs and symptoms occurring with hyperthyroidism, including tachycardia, palpitations and other cardiac dysrhythmias, angina, myocardial infarction, congestive heart failure, nervousness, tremors, hyperactivity, insomnia, diarrhea, abdominal cramps, nausea and vomiting, weight loss, fever, and intolerance to heat. Patient teaching for levothyroxine includes the following: For best absorption, take every morning on an empty stomach.
Do not take it with an antacid, iron preparation, or sucralfate. These drugs decrease absorption of levothyroxine. If one of these drugs must be taken, take the levothyroxine two hours before the other drug. Take your medication about the same time each day for more consistent blood levels and more normal body metabolism. Periodic tests of thyroid function are needed and may indicate adjustments in medication dosage. Do not switch from one brand name to another as the effects may be different. Levothyroxine stimulates the CNS and the heart.
Consult a health care provider before taking over-the-counter drugs that stimulate the heart or cause nervousness such as cold remedies, decongestants, asthma remedies. Also, limit intake of caffeine-containing drinks to 2-3 servings daily to avoid excessive stimulation. Report chest pain, heart palpitations, nervousness, or insomnia. These adverse effects are a result of excessive stimulation and may indicate the need to adjust drug dosage or decrease other stimulants. Levothyroxine therapy for hyperthyroidism is lifelong. Stopping it may lead to life-threatening illness.
When treating hyperthyroidism, the goals are to reduce the thyroid’s excess production of T4 and T3 hormones, restore TSH and thyroid hormones to normal, relieve symptoms, and avoid complete destruction of the thyroid gland. Antithyroid drugs include thioamide derivatives and iodine preparations. Thioamide derivatives include propylthiouracil (PTU) and methimazole. Iodine preparations include Lugol’s solution and SSKI (potassium iodide).
Thioamide drugs may be used as the primary treatment or to decrease blood levels of thyroid hormone before radioactive iodine therapy or surgery. A thioamide drug is given to produce a euthyroid state and an iodine preparation is then given to reduce the size and vascularity of the thyroid gland. This combination is given as a short-term treatment of hyperthyroidism, as in thyroid storm or in preparation for thyroidectomy. Propylthiouracil (PTU) is a thioamide derivative and the prototype of antithyroid drugs.
It works by inhibiting production of thyroid hormones and converting T4 to the more active T3. It is well absorbed when taken orally, but the duration of action is rather short, so it must be taken every 8 hours. The usual dose for adults is initially 300-400 milligrams per day until the patient is euthyroid. Then, it decreases to 100-150 milligrams per day for maintenance therapy. PTU is metabolized in the liver and excreted in the urine. It does carry an FDA black box warning regarding severe liver injury and acute liver failure.
PTU should be reserved for patients who cannot tolerate methimazole. Methimazole, or Tapazole, is similar to PTU and must also be taken orally every 8 hours. The patient is given 15-60 milligrams per day initially. Then, it decreases to 5-15 milligrams per day for maintenance. The rate of liver injury is considerably less with methimazole; however, it has shown three times more birth defects than PTU when taken in the first trimester of pregnancy. Therefore, PTU is the preferred drug of choice during the first trimester.
Patient teaching for PTU and methimazole include the following: Medications should be taken around the clock every 8 hours. Periodic tests of thyroid function and drug dosage adjustments are needed. It may be necessary to avoid or restrict iodide, such as seafood or iodized salt. Check with the physician. Report fever, sore throat, headache, skin rash, unusual bleeding or bruising, yellowing of the skin, or vomiting. These adverse effects may indicate the need to reduce drug dosage or discontinue the drug.
Consult a health care provider before taking any over-the-counter drugs, as some contain iodide, such as cough syrups, multivitamins, and asthma medications. These drugs can increase the risk of adverse effects and goiter. Radioactive iodine (I-131) is a frequently used treatment usually given in a single dose to destroy overactive thyroid tissue or to shrink a goiter. Large doses of I-131 are used to destroy thyroid cancer cells. Hypothyroidism is a common side effect and must be treated with thyroid hormone replacement.
Treatments with I-131 are generally safe, but patients need to follow radiation precautions after treatment to limit radiation exposure to others. Duration of precautions depends on the dose of treatment given and include limiting time in public places for 1-3 days, keeping at least six feet distance from others, do not prepare food for others or share utensils for 2-3 days, and sleep separately from another adult for 1-11 days and from a pregnant adult or child for 6-23 days. Antithyroid therapy is given in relatively large doses for 6-8 weeks.
Then, a maintenance dose is continued until the patient is euthyroid for 6-12 months. Therapeutic effects of antithyroid and iodine drugs include slower pulse rate, more normal activity level, less hyperactivity, slower speech, decreased nervousness, decreased tremors, weight gain, improved ability to sleep and rest. The adverse effects of PTU and methimazole include hyperthyroidism, blood disorders, skin rash, itching, hair loss, headache, dizziness, drowsiness, paresthesias, loss of taste, nausea and vomiting, abdominal discomfort.
Adverse effects of iodine preparations include a metallic taste, burning in the mouth, sore gums, excessive salivation, headache, rhinitis, red conjunctiva, edema of the eyelids, and hypersensitivity, such as acneiform, skin rash, itching, fever, jaundice, angioedema, and serum sickness. Let’s do a quick review. Thyroid hormones are required for normal growth and development and are considered especially critical for brain and skeletal development and maturation. Levothyroxine is the thyroid drug of choice for long-term treatment of hypothyroidism.
It is a synthetic preparation of T4 and must be taken for life. Antithyroid drugs used to treat hyperthyroidism include propylthiouracil (PTU), which is the prototype and can be used alone as pre-op preparation for thyroidectomy before or after radioactive iodine therapy and in treatment of thyroid storm. Other antithyroid drugs include methimazole and radioactive iodine. Patients should be monitored closely for hypothyroidism while taking antithyroid drugs.
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