logo

Anti-Thyroid Agents

methimazole (Tapazole ® ) propylthiouracil (PTU)

methimazole   (Tapazole ® ) top of page icon

CLINICAL PHARMACOLOGY
Methimazole inhibits the synthesis of thyroid hormones and thus is effective in the treatment of hyperthyroidism. The drug does not inactivate existing thyroxine and triiodothyronine that are stored in the thyroid or circulating in the blood nor does it interfere with the effectiveness of thyroid hormones given by mouth or by injection.

The actions and use of methimazole are similar to those of propylthiouracil. On a weight basis, the drug is at least 10 times as potent as propylthiouracil, but methimazole may be less consistent in action.

Methimazole is readily absorbed from the gastrointestinal tract. It is metabolized rapidly and requires frequent administration. Methimazole is excreted in the urine.

In laboratory animals, various regimens that continuously suppress thyroid function and thereby increase TSH secretion result in thyroid tissue hypertrophy. Under such conditions, the appearance of thyroid and pituitary neoplasms has also been reported. Regimens that have been studied in this regard include antithyroid agents as well as dietary iodine deficiency, subtotal thyroidectomy, implantation of autonomous thyrotropic hormone-secreting pituitary tumors, and administration of chemical goitrogens.


INDICATIONS AND USAGE
Methimazole is indicated in the medical treatment of hyperthyroidism. Long-term therapy may lead to remission of the disease. Methimazole may be used to ameliorate hyperthyroidism in preparation for subtotal thyroidectomy or radioactive iodine therapy. Methimazole is also used when thyroidectomy is contraindicated or not advisable.


CONTRAINDICATIONS
Methimazole is contraindicated in the presence of hypersensitivity to the drug and in nursing mothers because the drug is excreted in milk.

Dosing:
Adults: Initial: 15 mg/day for mild hyperthyroidism; 30-40 mg/day in moderately severe hyperthyroidism; 60 mg/day in severe hyperthyroidism; maintenance: 5-15 mg/day

Adjust dosage as required to achieve and maintain serum T3, T4, and TSH levels in the normal range. An elevated T3 may be the sole indicator of inadequate treatment. An elevated TSH indicates excessive antithyroid treatment.
--------------------------
Supplied:
Tablet: 5 mg, 10 mg, 20 mg

propylthiouracil   (PTU) top of page icon

CLINICAL PHARMACOLOGY:
Propylthiouracil inhibits the synthesis of thyroid hormones and thus is effective in the treatment of hyperthyroidism. The drug does not inactivate existing thyroxine and triodothyronine that are stored in the thyroid or circulating in the blood, nor does it interfere with the effectiveness of thyroid hormones given by mouth or by injection.

Propylthiouracil is readily absorbed from the gastrointestinal tract. It is metabolized rapidly and requires frequent administration. Approximately 35% of the drug is excreted in the urine, in intact and in conjugated forms, within 24 hours.

In laboratory animals, various interventions, including propylthiouracil administration, that continuously suppress thyroid function and thereby increase TSH secretion result in thyroid tissue hypertrophy.


INDICATIONS AND USAGE:
Propylthiouracil is indicated in the medical treatment of hyperthyroidism. Long-term therapy may lead to remission of the disease. Propylthiouracil may also be used to ameliorate hyperthyroidism in preparation for subtotal thyroidectomy of radioactive iodine therapy. Propylthiouracil is also used when thyroidectomy is contraindicated or not advisable.


CONTRAINDICATIONS:
Propylthiouracil is contraindicated in the presence of hypersensitivity to the drug or any of the other product components and in nursing mothers because the drug is excreted in milk.

--------------------------
Dosing:
Adults: Initial: 300 mg/day in divided doses every 8 hours. In patients with severe hyperthyroidism, very large goiters, or both, the initial dosage is usually 450 mg/day; an occasional patient will require 600-900 mg/day; maintenance: 100-150 mg/day in divided doses every 8-12 hours

Elderly: Use lower dose recommendations; Initial: 150-300 mg/day

Withdrawal of therapy: Therapy should be withdrawn gradually with evaluation of the patient every 4-6 weeks for the first 3 months then every 3 months for the first year after discontinuation of therapy to detect any reoccurrence of a hyperthyroid state.
--------------------------
Normal laboratory values:
Total T4: 5-12 mcg/dL
Serum T3: 90-185 ng/dL
Free thyroxine index (FT4 I): 6-10.5
TSH: 0.5-4.0 microunits/mL
-------------------------
Supplied:
Tablet: 50 mg
top of page icon

Disclaimer

Listed dosages are for - Adult patients ONLY. PLEASE READ THE DISCLAIMER CAREFULLY BEFORE ACCESSING OR USING THIS SITE. BY ACCESSING OR USING THIS SITE, YOU AGREE TO BE BOUND BY THE TERMS AND CONDITIONS SET FORTH IN THE DISCLAIMER. GlobalRPH does not directly or indirectly practice medicine or provide medical services and therefore assumes no liability whatsoever of any kind for the information and data accessed through the Service or for any diagnosis or treatment made in reliance thereon.

David F. McAuley, Pharm.D., R.Ph.  GlobalRPh Inc.
more Career Center image description
Medical Calculators - A thru Z
Lab Values - A thru Z