- Historically, hypothyroidism has been thought to interfere with fertility.
- Among women who were known to have thyroid deficiency, successful pregnancy was rarely reported during the first half of the 20th century, and miscarriage rates among such women were high.
- Even mild thyroid hormone deficiency in a mother may be harmful to the developing fetus. One study found if a mother has Free T4 levels that are less than the 10th percentile at 12 weeks gestation, the baby faces almost 6 times the risk to have impaired psychomotor development
- We now know that overt hypothyroidism in a pregnant woman, particularly during the first trimester, is associated with intellectual impairment in the child.
- Another study found that women who had elevated TSH levels during the second trimester of pregnancy had significantly reduced IQ in their children at the age of 7 and up.
Iodine and Pregnancy
According to research reported on at the May 2005 Endocrine Society annual meeting, the increased need for thyroid hormone seen in early pregnancy can only be met by increased hormone production by the gland, which depends on there being enough iodine available in the diet.
If there is insufficient iodine, the mother can become increasingly hypothyroid, and the thyroid increases in size (goiter).
The researchers concluded that iodine supplementation should be introduced to women during the early stages of pregnancy, in particular, multivitamins that contain iodine, to ensure that the total dietary intake of iodine is reaching approximately 250 g of iodine per day.
Source: Glinoer, D. "Pregnancy the Thyroid: Importance of the Iodine Nutrition Status." Endocrine Society Annual Meeting, May 2005.[S7-3]
Maternal Thyroid Hormone and the Developing Brain
Although it is known that thyroid hormone is needed for the developing fetal brain, scientists still do not know which developmental periods are most sensitive to thyroid hormone. The interplay between the fetal brain and the mother's thyroid hormone levels is still not clear. But understanding this connection better is critical, as this knowledge can help form the basis upon which doctors can treat hypothyroid women during pregnancy.
In an animal study reported on in the journal Endocrinology , hypothyroidism in pregnant rats was studied, and the mother rat's thyroid function was restricted for only a few days during a time period before the fetal thyroid gland had a chance to develop. This gave the researchers a chance to see the effects of the maternal thyroid hormone on the offspring. They found that the offspring of hypothyroid mothers had misplaced neurons in the brain, and were prone to seizures. The The study's authors concluded that thyroid function should be monitored carefully from the earliest stages of pregnancy to avoid brain damage in the fetus, since even mild fluctuations can affect the developing brain.
Source: Forrest, Douglas, "News & Views: The Developing Brain and Maternal Thyroid Hormone: Finding the Links," Endocrinology 145 (9):4034-4036, 2004.
Thyroid Function Tests & Pregnancy: What's Normal?
At the Endo 2005 conference, research was presented that looked at the body's response to the increased demand that pregnancy puts on the mother's thyroid production. According to the researchers, non-pregnant reference ranges are "unreliable for assessing thyroid status during pregnancy."
At the same time, however, pregnancy-specific reference ranges have not been done, because, according to the researchers, "many studies have been compromised by iodine deficiency and/or the failure to exclude individuals with autoimmune thyroid disease."
This has led the researchers to conclude the following:
There is growing consensus that a first trimester serum TSH above 2.5 mIU/L may indicate thyroxine insufficiency, especially if TPOAb (thyroid peroxidase antibodies) is detected." "The presence of Thyroid Peroxidase Antibody (TPOAb) is an important marker of AITD. A detectable pre-pregnancy TPOAb is a risk factor for complications resulting from thyroid insufficiency during pregnancy and post-partum thyroiditis.