Soy Formula Makes Congenital Hypothyroidism Harder to Treat



By Anthony J. Brown, MD

NEW YORK (Reuters Health) Feb 25 - The elevated TSH levels seen with
congenital hypothyroidism (CH) respond to levothyroxine more slowly when
the infant is being fed soy formula rather than other diets, new research
indicates. Close thyroid monitoring is a must in soy-fed infants and the
levothyroxine dose may need to be increased, say the authors of the study.

"Having normal thyroid function tests in infancy is critical for normal
brain development," lead author Dr. S. C. Conrad, from Children's Hospital
Oakland in California, told Reuters Health. "So, if something is
interfering with this, it can cause significant long-term problems."

Recent case reports have suggested that the use of soy formulas in CH
patients may slow the resolution of abnormal thyroid function tests. "For
this reason and because such formulas are becoming increasingly popular, we
decided to look at the association in our patient population," Dr. Conrad
said.

As reported in the Archives of Disease in Childhood for January, the
researchers assessed a variety of TSH parameters in 8 CH patients fed a soy
diet and 70 CH patients fed a non-soy diet. The two groups were comparable
in terms of the starting levothyroxine dose and the change in this dose
over one year.

All of the measures pointed toward a prolonged increase of TSH in the soy
diet group, the investigators note. For example, normalization of TSH took
a median of 150 days in the soy diet group compared with just 40 days in
the control group (p = 0.02). Similarly, by 4 months, 62.5% of soy-fed
infants still had an elevated TSH level compared with only 17% of controls
(p = 0.01).

"Clinicians treating soy-fed CH patients need to be a little bit more
vigilant about checking thyroid function tests and be aware that a higher
levothyroxine dose may be required," Dr. Conrad said.

In terms of the mechanisms involved, Dr. Conrad said that animal studies
have shown that soy interferes with the intestinal absorption of
levothyroxine, resulting in fecal loss of the drug.

Arch Dis Child 2004;89:37-40.

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