TUESDAY, Aug. 21, 2012 (HealthDay News) -- Babies born with congenital hypothyroidism -- a condition that causes low or no thyroid hormone production -- shouldn't be given soy formula, new research indicates.
Soy can interfere with the absorption of levothyroxine, the medication that replaces the missing thyroid hormone. If babies and toddlers don't get enough replacement thyroid medication, their brains can't develop properly, experts note.
Two case studies that highlight the importance of this issue are detailed by doctors from the University of California, San Diego, in the September issue of the journal Pediatrics.
"Soy products interfere with levothyroxine absorption and endanger infants and young children with congenital hypothyroidism who are at risk for developmental and growth delay," wrote the authors of the case studies.
"When a child is hypothyroid at birth, it's really a medical emergency," said Dr. Brenda Kohn, director of pediatric endocrinology and director of the neonatal thyroid center at New York University Langone Medical Center in New York City. "The impact of soy formula is dramatic in the neonate, and it should be avoided."
Kohn said the issue of soy decreasing the absorption of levothyroxine (brand name Synthroid, among others) is one that's well known to pediatric endocrinologists, but pediatricians and parents may not be as familiar with the potential dangers.
The first case study was an infant girl who was diagnosed with congenital hypothyroidism from the newborn screening test. She was started on levothyroxine when she was six days old. The baby's older brother had been lactose-intolerant, so to avoid that issue the parents started the baby on a soy formula. They gave her levothyroxine about an hour after she had eaten. At three weeks, her pediatrician noticed that the baby had lost weight and didn't have the right muscle tone. When blood work was done, she was found to have very little thyroid hormone. After stopping the soy formula, the baby quickly recovered and had no lasting damage.
The second case study was a 5-year-old girl. She also was diagnosed through newborn screening and quickly started on levothyroxine. At age 5, she was showing signs of hypothyroidism despite a high dose of levothyroxine. Her height had dropped from the 25th percentile to the ninth, but her weight was in the 55th percentile. She wasn't able to identify numbers or letters at the time. Doctors soon learned that she had been drinking 8 ounces of soy milk daily. She usually took her levothyroxine at least an hour after drinking soy milk, according to the case study. But when the soy milk was stopped, her thyroid levels improved dramatically, and her weight dropped to a more expected level.
"Hypothyroid in babies is easy to treat, but devastating not to treat," said Dr. Rubina Heptulla, chief of pediatric endocrinology at the Children's Hospital at Montefiore in New York City.
The good news, she said, is that even if babies have been given soy, regular blood tests should catch any drop in thyroid levels before damage is done.
"If the baby is being monitored closely, a decrease would get picked up," she said. "Symptoms would not appear."
Both Heptulla and Kohn said parents should be told when their baby is diagnosed with hypothyroidism that they need to avoid soy products, especially in infancy and the preschool years. For older children and adults, it's probably OK to have soy once in while as long as it's not eaten around the time you're taking your medication, Kohn said. But, she noted, it's a good idea to avoid soy for the most part.
She also said babies generally should be on low-iron formula, because iron can affect the absorption of levothyroxine too. If your child has been prescribed iron drops or multivitamins containing iron, they shouldn't be given at the same time as levothyroxine.
Learn more about congenital hypothyroidism from the OrphaNet Journal of Rare Diseases.