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Soy Formulas and the Effects of Isoflavones on the Thyroid

In a paper published in the New Zealand Medical Journal (Volume 113, Feb 11, 2000), environmental scientist and long-time campaigner against soy-based infant formulas, Dr Mike Fitzpatrick, has warned about the risk of thyroid disease in infants fed soy formulas, high soy consumers and users of isoflavone supplements:

"There is potential for certain individuals to consume levels of isoflavones in the range that could have goitrogenic effects. Most at risk appear to be infants fed soy formulas, followed by high soy users and those using isoflavone supplements".

The report noted that infants fed soy formulas are exposed to high levels of isoflavones, which are potent anti-thyroid agents, and that the risks to normal growth and development were significant. Fitzpatrick stated that thyroid problems due to soy might not be recognised "due to difficulties in establishing a cause and effect relationship" and noted that even experienced soy researchers may be ignorant of the connection between isoflavones and goitre. Fitzpatrick also rejected claims that there was no evidence that isoflavones in soy formulas harmed infants citing the reported cases of goitre that have occurred in infants fed iodine sufficient soy formulas.

Fitzpatrick stated his support to the position of the New Zealand Ministry of Health:

"MOH has found that infants with a history of thyroid dysfunction should avoid soy formulas and soy milks. Additionally, there is potential for isoflavone exposure to cause chronic thyroid damage in all infants fed soy formulas"

Fitzpatrick stated that exposing infants to isoflavones was unnecessary and that the risk of harm could be avoided if manufacturers removed isoflavones from soy formulas. "In the interim" he stated "it is appropriate for medical practitioners to monitor the thyroid status of infants fed soy formulas"

Fitzpatrick also claimed that high soy consumers and users of isoflavone supplements were also at risk of thyroid disorders. He stated that the subtle effects of anti-thyroid agents on thyroid function would most likely be evidenced as subclinical, or even overt hypothyroidism.

Fitzpatrick also noted that a sporadic pattern of soy use may also not be without risk since the resulting thyroid stimulation parallels the classic method for inducing thyroid tumours in laboratory animals. He recommended "a more cautionary approach to the use of soy and isoflavone supplements".

Download the full paper