When a woman becomes pregnant, many changes occur in her body. One of those changes is in the levels of various hormones produced by the body, including those produced by the thyroid.
In the case of thyroid-stimulating hormone (TSH), pregnant women typically produce a lower level than normal (0.4-4.0 milli-international units per liter). Some international guidelines recommend levels be no higher than 2.5-3. milli-international units per liter during pregnancy. When their TSH levels rise above this, they may experience subclinical hypothyroidism, or mildly underactive thyroid, which can cause a number of health problems if left untreated.