According to an article appearing in the New Zealand Medical Journal (volume 113 (1103):24-6, Feb 11, 2000), consumption of high levels of soy products may have an adverse effect on the thyroid. The group that the author was most concerned about was infants who are fed soy formula on a daily basis.
The author stated his position of the New Zealand Ministry of Health: “The Ministry of Health 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” The author’s positions is that isoflavones are 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”
There is some controversy with this position. Some experts believe that soy formula that is fortified with iodine mitigates this effect. The author also concedes that many researchers do not see the connection between high soy consumption and thyroid problems because of the difficulty in establishing a cause-effect relationship.