Introduction

Thyroid disease is the most common Endocrine disorder in women and the most common Endocrine disorder in pregnancy. Certain thyroid disorders may have serious impact on the mother and her unborn child.

The thyroid gland synthesizes the thyroid hormone through utilization of ingested Iodine, which is responsible for a variety of actions in the human body. For example, the thyroid hormone is responsible for maintaining the metabolic functions of the body and regulating oxygen consumption. Thyroid gland abnormalities could be either hyperactive (excess in function) or hypoactive (less function). Derangement in Iodine supply could lead to abnormality in the size (goiter) and/ or function of the thyroid gland.

Background

Changes in thyroid functions during pregnancy

Pregnancy has significant impact on the regulation of thyroid hormone; the total amount of thyroid hormone (Thyroxin or T4) increases due to increase in the Thyroid Binding Globulin (TBG). There is increase in the renal clearance of Iodine and increase transfer of iodine and thyroid hormone from the mother to the developing fetus. However, the active form of thyroid hormone (Free T4) remains unchanged.  Iodized salt is sufficient to supply Iodine during pregnancy and prenatal Vitamins ensure adequate Iodine intake.

Thyroid Disorders

Hypothyroidism, or under active gland, is usually sub-clinical in pregnancy. Most patients with this condition have a history of thyroid surgery (thyroidectomy) or a history of hyperactive thyroid (Graves' thyrotoxicosis). Profound hypothyroidism has been associated with poor pregnancy outcome, like preeclampsia, stillbirth, and other maternal complications.

Hyperthyroidism, or Grave's disease, accounts for the majority of hyperthyroidism in pregnancy. Grave’s disease is an autoimmune disorder that is characterized by a large thyroid gland (Goiter), tachycardia, heat intolerance, hypertension, anxiety, exophthalmia, and laboratory evidence of excess thyroid hormone. Grave's disease may have significant maternal and fetal complications if left untreated, and presents a 1% risk of congenital malformation.

Thyroid Storm: This is a real life-threatening emergency; it is characterized by coma, cardiac arrhythmia, and high fever. Patient at risks who have infections, trauma, surgery and preeclampsia.

Postpartum Thyroiditis: Complicates 5-10% of pregnancies and may have Biphasic course; hyperthyroid phase followed by hypothyroid phase. Recovery is anticipated in 90% of patients who will have normal thyroid functions. In some patients, permanent hypothyroidism may be present.

Congenital Hypothyroidism: This is a rare condition that could lead to severe intellectual impairment if the newborn is left untreated.

Treatment

Hypothyroidism:  Replacement of the deficient thyroid hormone is the mainstay of management. Thyroid hormone in the form of L-thyroxin, 100ug a day can be safely given during pregnancy. Dosing adjustment may be necessary in pregnancy to maintain adequate replacement.

Hyperthyroidism:  Anti thyroid drugs can be used with minimal risks to the fetus. Propylthiouracil (PTU), prevents T4 synthesis, and is the most commonly used anti-thyroid medication in pregnancy. Even though the medication does cross the placenta, the risk of congenital defects is minimal. There are rare maternal side effects like nausea, skin rash, fever, and agranulocytosis.  Thionamide Methimazole (Carbazol) is equally effective compared with PTU but more likely to cross the placenta.  Carbazol is usually used when PTU is not tolerated.

Anti-thyroid therapy has been associated with rare incidences of fetal hypothyroidism.  Anti-thyroid therapy is compatible with breast-feeding.

Postpartum thyroiditis:  In severe cases of postpartum thyroiditis Beta-blockers are administered.

Surgery:  Surgery is indicated when medical therapy is ineffective or intolerable.  The second trimester is the preferred time for surgery.

Treatment of thyroid storm: Thyroid storm treatment requires hospitalization in an intensive care setting. Usually a combination of anti-thyroid drugs and sodium Iodide will control the patient's condition.

References:

Hall et al, Br J OB/GYN 1993

Weeke et al, Acta Endocrinol, 1982

OB/GYN Clinics of North America, 9/1997

Contemporary OB/GYN, 10/1999

Medical Complication during Pregnancy, Burrow, 5^th Edition, Saunders 1999