In pregnancy, the physiologic functions of the mother prior to conception are altered to provide the needs of the fetus growing in her womb. The placenta itself starts producing several hormones that are vital for a safe and normal pregnancy. These changes may also affect thyroid function and thyroid screening tests.
One of these changes is the increase in renal blood flow and the filtration rate of the kidneys, leading to an increased secretion of the circulating iodides in the body. Normally, most of the iodides acquired through ingestion of iodine-rich foods are quickly excreted by the kidney but not before the thyroid gland manages to “trap” about a fifth of the circulating iodides for the production of thyroid hormones. Because normal physiologic changes gets rid of some of the iodides before the thyroid manages to trap them, dietary iodide requirements are raised to around 200 micrograms per day in the pregnant woman in contrast to only 150 micrograms per day for normal persons.
In pregnancy, the motherÂ’s thyroid gland ordinarily enlarges up to 50 percent with a corresponding increase in thyroxine or T4 secretion. One of the reasons for this change is that the human chorionic gonadotropin (hCG) secreted by the placenta has a stimulating effect on the thyroid. There is a broad structural homology between the beta subunits of hCG and thyroid-stimulating hormone (TSH), which gives hCG a weak thyroid-stimulating effect. Another hormone which may also affect the increase in thyroxine production by the mother is human chorionic thyrotropin, which is also secreted by the placenta.
Thyroid Function in the Fetus
The thyroid gland of the fetus begins to function by the end of the third month of gestation but prior to that time, the fetus is largely dependent on maternal thyroid hormones. The fetal thyroid gland gains the ability to produce thyroid hormones by around the eighth to tenth week of gestation. In the twelfth week of gestation, it begins to exhibit the ability to trap iodine actively and begins to produce thyroxine soon after.
Thyroid hormone is vital for fetal brain development as in neuronal multiplication, migration, and the structural organization of the brain of the fetus. Brain development occurs mostly during the second trimester, wherein the fetus is yet unable to produce its own fetal thyroid hormones and is largely dependent on maternal thyroid hormones for normal brain development.
Evidence supports the transfer of maternal thyroid hormones to the fetus via the placenta before and after fetal thyroid functionality. Thirty percent of the thyroxine found in the cord blood of neonates is maternal thyroxine and it has been found in the amniotic sacs of fetuses from 4 months to 6 months in gestation.
Thyroid Problems in the Newborn
The endocrine system of the newborn is usually highly developed at the time of birth and the neonate rarely exhibits thyroid function problems immediately. However, in some instances, thyroid function in the infant must be monitored.
One such example is that if the mother experienced hyperthyroidism or was treated with excess thyroid hormone during her pregnancy. If such is the case, the infant may be born with a thyroid gland that secretes less than the normal amount of thyroid hormone due to the excess in maternal thyroid hormone.
On the other side, if the mother had a thyroidectomy or surgical removal of her thyroid gland, the child may be born with temporary hyperthyroidism. This may be because the pituitary gland of the mother secretes increased amounts of thyroid-stimulating hormone due to her hypothyroid state. The thyroid gland of the fetus then responds to the maternal TSH and starts producing thyroid hormone in excess.
In a fetus with a problem in thyroid hormone secretion, there is poor bone development and mental retardation. This may cause the condition cretin dwarfism. If the newborn is not treated within several weeks, mental retardation becomes permanent. It is for this reason that newborns are routinely screened for hypothyroidism so as to prevent the devastating and permanent effects of cretinism.
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