Pregnant women with high levels of thyroid hormone may pass on toxins to their developing fetus

Aug 11 2004

A study published in the current issue of the Journal of the American Medical Association reports that pregnant women with high levels of thyroid hormone may pass on toxins to their developing fetus.

Thyroid hormones are a class of hormones, synthesized by follicular cells within the thyroid gland. The thyroid gland is a small, butterfly-shaped gland located at the base of the neck over the trachea, or windpipe. Its job is to extract iodine from blood to produce two hormones - thyroxine and triiodothyronine - that regulate the energy use of virtually every cell and organ in the body.

Maternal hypothyroidism (an under active thyroid gland) and hyperthyroidism (an overactive thyroid gland) have harmful effects on the outcome of pregnancy. While the effects of TH deprivation on the fetus, independently from that on the mother, can be studied in infants with congenital hypothyroidism, this is not the case in those with fetal thyrotoxicosis (an overactive thyroid gland).

Hyperthyroidism occurs seven to nine times more likely in women than in men. Hyperthyroidism often occurs between the ages of twenty and forty, while hypothyroidism often appears after the age of fifty.

Thyroid hormones play an important role in the development of the embryo and fetus. Deficiency of thyroid hormone secretion (an under active thyroid gland) during fetal development or early infancy results in infantile cretinism (congenital hypothyridism). Respiratory difficulties, persistent jaundice, and hoarse crying in infants; stunted growth (dwarfism), bone and muscle dystrophy, and mental deficiency in older children.

João Anselmo, M.D., of Hospital Divino Espírito Santo, Ponta Delgada, Azores-Portugal, and colleagues from the University of Chicago Hospitals studied the effects of TH excess on fetuses carried by mothers who, because of their resistance to TH (RTH) are normal despite high TH levels but who carry normal fetuses that have been exposed to high maternal hormone levels. The study included 167 members of an Azorean family with RTH. Affected individuals had the RTH phenotype.

Thirty-six couples with complete information belonged to 1 of 3 groups: affected mothers (n=9), affected fathers (n=9), and unaffected relatives (n=18). "Mean miscarriage rates were 22.9 percent, 2.0 percent, and 4.4 percent, respectively. Affected mothers had an increased rate of miscarriage. They had marginally higher than expected numbers of affected offspring, i.e., 20 affected and 11 unaffected children, while affected fathers had 15 affected and 12 unaffected children. Unaffected infants born to affected mothers were significantly smaller than affected infants...," the authors write. "Our data show a 3- to 4-fold increase in the rate of miscarriage in affected mothers compared with that of spouses of affected fathers or unaffected first-degree relatives..."

"The data presented herein show, for the first time in humans, that high levels of TH can exert a direct toxic effect on fetal development. This is manifested by an increased rate of miscarriages and a lower birth weight of unaffected infants born to euthyroid mothers with high levels of TH. As expected, fetuses harboring a mutation that reduces the sensitivity to TH are protected from this toxic effect of TH excess. Given the established importance of providing TH replacement to even mildly hypothyroid pregnant women, it is important to recognize that over replacement appears to be equally detrimental," the authors conclude.

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