Thyroid disorders are often present during pregnancy. High thyroxine levels are often caused by hyperthyroidism or thyroiditis (see Thyroid disorders). Hyperthyroidism is caused by an antibody that stimulates the thyroid gland to produce too much thyroxine. These antibodies can pass through the placenta and promote the activity of the fetal thyroid gland, causing an increased fetal heart rate (more than 160 beats per minute) and slowed growth. Sometimes, hyperthyroidism can produce antibodies that prevent the production of thyroxine. These antibodies can inhibit the fetus from producing enough thyroxine through the placenta, and low thyroxine causes a mental retardation in the fetus called cretinism. There are many treatment options for hyperthyroidism. The lowest effective dose of antithyroid medication is usually given. Careful monitoring is required when administering the medication, as this drug can pass through the placenta and prevent the fetus from producing sufficient amounts of thyroxine. Hyperthyroidism improves in the last trimester of pregnancy, when the dose of antithyroid medication can be reduced or stopped altogether. An experienced thyroid surgeon can remove the thyroid gland in the middle of pregnancy (4th to 6th trimester), but that pregnant woman must be started on thyroxine 24 hours after surgery and remain on the medication for life. The amount of thyroxine is only enough to replace the amount produced by the normal thyroid gland. Therefore, it will not cause other problems in the fetus. Thyroiditis is an inflammatory condition of the thyroid gland with mild enlargement of the thyroid gland. It can also cause some temporary symptoms during pregnancy due to a temporary increase in thyroxine levels. It usually does not require treatment. During the first few weeks after delivery, a painless thyroiditis with a temporary increase in thyroxine production can occur suddenly. These symptoms may persist or worsen, sometimes with repeated transient elevations of thyroxine. One of the most common conditions that cause a decrease in thyroxine levels during pregnancy is Hashimoto’s thyroiditis (see Thyroid disease), often caused by an antibody that prevents the production of thyroxine. Its treatment is similar to that of hyperthyroidism. Sometimes the course of Hashimoto’s thyroiditis may improve temporarily during pregnancy. However, pregnant women with low thyroxine levels should be supplemented with oral thyroxine. Thyroxine levels are measured after a few weeks so that the dose can be adjusted. Dose adjustment is also required as the pregnancy progresses. Thyroid dysfunction occurs in 4-7% of women during the first 6 months after delivery. These women often have a family history of thyroid disease, diabetes, or she has a pre-existing thyroid condition such as goiter or Hashimoto’s thyroiditis, all of which are particularly prone to postpartum hypothyroidism. Low or high thyroid levels that occur after pregnancy are temporary, but require treatment.