Munro IC et al
Negative Impact of Soy-Based Infant Formula on Thyroid Function: Human Case Reports
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During the 1950s and 1960s, 12 cases were reported where infants fed soy-based infant formula exhibited changes in thyroid function. The majority of these cases were linked to goitre, though no symptoms of hypothyroidism (thyroid dysfunction caused by iodine deficiency) were observed.
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Further research revealed that under iodine-deficient dietary conditions, genistein and daidzein—two isoflavones found in soy—inhibited thyroid peroxidase (TPO) activity, an enzyme crucial for the synthesis of iodothyronine hormones (T3 and T4). This discovery led to the mandatory supplementation of soy-based infant formula with iodine starting in the 1960s.
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A specific clinical case also highlighted concerns about soy-based infant formula. A male infant diagnosed with congenital hypothyroidism began treatment with thyroxine (T4) at 11 days old, but his hypothyroidism persisted until his diet was switched to cow’s milk.
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These findings suggest that while iodine supplementation helped mitigate some risks, soy-based infant formulas may still affect thyroid hormone metabolism, particularly in iodine-deficient conditions or infants with underlying thyroid issues.
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