With two rare exceptions, discussed further on, the causes of thyroid disease during pregnancy are the same as in the general population. The most common thyroid diseases in pregnancy mirror the most common thyroid diseases in the general population. As in the general population, Hashimoto's disease (see Chapter 5) is the most common thyroid disease in pregnancy, followed by Graves' disease (see Chapter 6). In both cases, the risk spikes during the first three months of pregnancy, and then spikes again in the first six months after delivery. Many women will first develop Hashimoto's or Graves' disease within a year of their pregnancies. After delivery, up to 20 percent of all women (particularly those with thyroid antibodies or insulin-dependent diabetes) will develop postpartum thyroiditis, which usually resolves on its own but 25 percent of the time can leave women permanently hypothyroid.
Just as in the general population, pregnant women can develop hypothyroidism or thyrotoxicosis for other reasons, discussed in Chapters 3 and 4. But what does seem clear (at least to thyroid researchers) is that pregnancy increases your body's thyroid hormone requirements. Pregnancy may lead to relative iodine deficiency, which can increase the severity of preexisting hypothyroidism, and worsen preexisting Hashimoto's or Graves' disease. Finally, it can unveil overt hypothyroidism in women who had subclinical hypothyroidism prior to pregnancy. In some rare cases, if there is a great deal of bleeding during delivery, the pituitary may be damaged (Sheehan's syndrome) and result in hypothyroidism, loss of cortisol from the adrenal gland, and infertility.
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