ATA News Release 2004

Infants of mothers with over- or under-active thyroid may have vision processing problems

(VANCOUVER, BC, Sept. 30, 2004)—Children of women with an underactive thyroid — called hypothyroidism — and an overactive thyroid — called hyperthyroidism — have subtle deficits in visual processing despite the mothers’ treatment, according to a new study being presented on Thursday, Sept. 30, at the 76th Annual Meeting of the American Thyroid Association in Vancouver, British Columbia, Canada. These visual processing problems include difficulty distinguishing between colors in the same family of colors and impaired contrast sensitivity, of which both problems can affect certain learning activities in school as well as professional choices later in life.

“In the case of maternal hypothyroidism, treatment for the mother was not enough to prevent these visual processing problems,” said Joanne Rovet, PhD, Senior Scientist at The Hospital for Sick Children in Toronto and Professor of Pediatrics at University of Toronto. “In maternal hyperthyroidism, either the mother’s treatment blocked or the mother’s antibodies interfered with fetal thyroid functioning.” Dr. Rovet is the principle investigator for this study, which is funded by the Canadian Institutes for Health Research.

Animal research has shown that thyroid hormone is essential for the developing visual system. Specifically, thyroid hormone is involved in the formation of the neural pathway that underlies motion and contrast detection as well as the development of retinal cones integral for color vision. Past research has shown that offspring of hypothyroid women display subtle visual deficits in contrast detection. However, it was unknown whether similar or different deficits occur in offspring of hyperthyroid women or whether either group has a color vision deficit.

Contrast detection develops before color vision, therefore, Dr. Rovet and her colleagues hypothesized that hypothyroid women would show contrast sensitivity defects, whereas hyperthyroid women would show color vision defects. In this ongoing study, they have so far examined 42 6–8 month-old infants of women with hypothyroidism, hyperthyroidism, and normal thyroid hormone function. Children of both hypothyroid and hyperthyroid mothers were less able to make subtle distinctions between colors and had weaker contrast sensitivity. The extent of the color deficit appeared to be stronger in the children of hypothyroid mothers. The researchers believe that their findings support the view that the developing visual system is vulnerable to a gestational lack of thyroid hormone and that the type and extent of deficit reflects the timing and nature of the maternal thyroid problem.

Based on these findings, the researchers advise that mothers with pre-existing hypothyroidism should be monitored closely and treated according to their pregnancy with a dose of about 150 percent of prepregnancy levels. Women at high risk based on symptoms and family history should have thyroid hormone levels checked pre-pregnancy and in early pregnancy and treated as soon as thyroid-stimulating hormone levels rise. If a woman develops hypothyroidism in pregnancy, it should be treated quickly. “All women of reproductive age should be made aware of mild symptoms and have thyroid hormone levels checked if any symptoms appear,” said Dr. Rovet.

Currently, there is no treatment for these visual processing problems, said Dr. Rovet. “It’s important that parents and teachers understand this issue so visual formats can be modified for the children,” she added. “For example, teachers can make sure that everything is presented in high contrast and not make normal assumptions about colors.”

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