Thyrotoxicosis during pregnancy may result in fetal anomalies, late abortion, or preterm labor and fetal hyperthyroidism with goiter. Thyroid storm in late pregnancy or labor is a life-threatening emergency.
Radioactive isotope therapy must never be given during pregnancy. The thyroid inhibitor of choice is propylthiouracil, which acts to prevent further thyroxine formation by blocking iodination of tyrosine. There is a 2- to 3-week delay before the pretreatment hormone level begins to fall.
The initial dose of propylthiouracil is 100-150 mg three times a day; the dose is lowered as the euthyroid state is approached. It is desirable to keep free T4 in the high normal range during pregnancy. A maintenance dose of 100 mg/d minimizes the chance of fetal hypothyroidism and goiter.
Recurrent postpartum thyroiditis occurs 3-6 months after delivery. A hyperthyroid state of 1-3 months’ duration is followed by hypothyroidism, sometimes misdiagnosed as depression. Thyroperoxidase antibodies and thyroglobulin antibodies are present. Recovery is spontaneous in over 90% of cases after 3-6 months.
Maternal hypothyroidism - even subclinical hypothyroidism manifested only by elevated levels of thyroid-stimulating hormone (TSH) - may adversely affect subsequent neuropsychologic development of the child. Mothers with known or suspected hypothyroidism should have the TSH level measured at the first prenatal visit. Replacement therapy with levothyroxine should be adjusted to maintain levels of TSH in the normal range.
Lazarus JH et al: Thyroid disease in relation to pregnancy: a decade of change. Clin Endocrinol 2000;53:265.
Revision date: June 21, 2011
Last revised: by Jorge P. Ribeiro, MD