Medication Summary
The goals of pharmacotherapy are to reduce morbidity and to prevent complications.
Antithyroid drugs
Class Summary
ATDs are effective, reversible treatments for hyperthyroidism. The consensus among experts is that ATDs should be first-line treatments for pregnant women with hyperthyroidism. ATDs inhibit iodination of thyroid thyroglobulin and thyroglobulin synthesis by competing with iodine for peroxidase. PTU and MMI are available in the United States. PTU and MMI are equally effective.
Both PTU and MMI cross the placenta and can cause fetal hypothyroidism and goiter. In addition, both PTU and MMI are excreted in small amounts in breast milk. MMI is not bound to plasma protein as much as PTU; therefore, it is excreted into the breast milk in slightly higher concentrations than PTU is.
Propylthiouracil
Derivative of thiourea that inhibits organification of iodine by thyroid gland. Blocks oxidation of iodine in thyroid gland, inhibiting thyroid hormone synthesis. Inhibits T4-to-T3 conversion (advantage over other agents). DOC in patients with hyperthyroidism during pregnancy because of reports of fetal aplasia cutis (reversible scalp defect) in association with MMI or CMI. Taper gradually to minimum dosage required to maintain clinical euthyroid and to avoid fetal hypothyroidism.
Methimazole, MMI (Tapazole)
Inhibits thyroid hormone by blocking oxidation of iodine in thyroid gland, but not known to inhibit peripheral conversion of thyroid hormone. Taper gradually to minimum dosage required to maintain clinical euthyroid and to avoid fetal hypothyroidism. Cases of fetal aplasia cutis reported.
Iodides
Class Summary
Iodides inhibit the release of stored thyroid hormones. They decrease serum T4 and T3 concentrations by 30-50% by the 10th day of treatment. Iodides are reserved for the treatment of severe thyrotoxicosis or for preoperative treatment in combination with ATDs and beta-blockers. These agents readily cross the placenta by the 12th week of gestation and are readily taken up by the fetal thyroid gland. Long-term use of iodides can lead to fetal hypothyroidism and goiter. In pregnant women, these drugs should generally be used for no longer than 2 weeks.
Iodide (SSKI, Pima)
DOC. Rapidly inhibits release of thyroid hormones by directly affecting thyroid gland. Inhibits synthesis of thyroid hormones. Also appears to attenuate cyclic adenosine monophosphate (cAMP)–mediated effects of TSH.
Thyroid hormones
Class Summary
Several forms of thyroid hormones are commercially available. They include levothyroxine, liothyronine, and liotrix. Levothyroxine is the DOC for treating pregnant women with hypothyroidism. No conclusive evidence supports the use of levothyroxine to prevent perinatal hypothyroidism.
Levothyroxine (Synthroid)
Levo isomer of T4. Once absorbed, T4 deiodinated to T3 in extrathyroidal tissues. First choice in treatment of hypothyroidism during pregnancy because it mimics physiologic state. Measure TSH levels q4wk, and adjust dosage.
Beta-adrenergic blocker
Class Summary
Beta-blockers are valuable adjunct to ATDs. These drugs are effective for alleviating symptoms of hypermetabolic state (eg, palpitation, sweating, nervousness, tremor). They are safe during pregnancy to achieve immediate control of symptomatic thyrotoxicosis. The goal of beta-blockage is to reduce the mother's heart rate to less than 100 bpm. Prolonged use of beta-blockers has been associated with intrauterine growth restriction, fetal bradycardia, hypoglycemia, and an abnormal response to stress; therefore, long-term use not recommended during pregnancy.
Propranolol (Inderal)
DOC in treating cardiac arrhythmias resulting from hyperthyroidism. Controls cardiac and psychomotor manifestations in minutes.
Tables
Serum TSH level, mIU/mL or mIU/L |
Increase, mcg/d |
5-10 |
25-50 |
10-20 |
50-75 |
> 20 |
75-100 |
