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Consuming Soy and Thyroid Problems Are Not Correlated
One possible concern with soy consumption is the potential association
between dietary soy and thyroid health. Early studies on soy and
thyroid function reported that rodents fed soy-based diets had
a high incidence of goiter (Medical References 1-3); however,
these soy and thyroid problems encountered were due to iodine-deficient
diets (Medical References 4, 5). Similar problems were
observed in infants fed soy-based formulas that were not fortified
with iodine (Medical References 6-10). Since the development
of soy-based infant formulas using more refined soy protein and
supplemented with iodine, soy and thyroid problems in infants have
been non-existent.
Hypothyroidism risk also has been a concern in regards to soy
and thyroid health. Animal and cell culture studies on soy and
thyroid function reported that soy isoflavones inhibit the activity
of thyroid peroxidase (Medical References 11-13), the
enzyme responsible for thyroid hormone production. Despite this
enzyme inhibition, consumption of soy isoflavones does not cause
hypothyroidism, since the thyroid hormones and the thyroid gland
were unaffected in rats (Medical References 12, 13).
Studies on soy and thyroid health in human subjects confirmed
the findings reported in the preclinical studies regarding thyroid
hormone effects. While soy consumption by men with slightly high
cholesterol increased T4 levels, these levels remained within the
normal range (Medical Reference 14). In premenopausal
women, consumption of soy with different soy isoflavone levels
for 3 months had no effect on serum levels of TSH, total and free
T4, and total T3, though a modest reduction in free T3 was observed
(Medical Reference 15). Similarly, studies on soy and
thyroid health in postmenopausal women indicate that no or only
modest changes in thyroid hormone levels have been observed in
association with soy consumption (Medical References 16-18).
While soy does not adversely affect thyroid health in healthy,
iodine-replete individuals, there may be subpopulations where soy
consumption may be a concern. It has been reported that soy infant
formula can increase TSH levels and affect management of hypothyroidism
in infants with congenital hypothyroidism (Medical References
19, 20). Additionally, consumption of soy-based foods appears
to effect the absorption of synthetic T4, so it is necessary either
to alter the dose of medication or separate soy consumption and
medication intake by a few hours so as not to interfere with absorption
(Medical References 19, 21, 22).
The research on soy and thyroid health indicates that soy does
not adversely affect thyroid hormone levels or the thyroid gland
in healthy individuals consuming an iodine-sufficient diet. Overall,
these data indicate that soy and thyroid health are not negatively
correlated.
Medical References
- McCarrison R. The goitrogenic action of soya-bean and ground-nut.
Ind J Med Res 1933; XXI:179-181.
- Sharpless GR, Pearsons J, Prato GS. Production of goiter in
rats with raw and withtreated soybean flour. J Nutr 1939;
17:545-555.
- Wilgus HS, Jr., Gassner FX, Patton AH, Gustavson RG. The goitrogenicity
of soybeans. J Nutr 1941; 22:43-52.
- Block RJ, Mandl RH, Howard HW, Bauer CD, Anderson DW. The curative
action of iodine on soybean goiter and the changes in the
distribution of iodoamino acids in the serum and in the thyroid
gland digests. Arch Biochem Biophyics 1961; 93:15-21.
- Kay T, Kimura M, Nishing K, Itokawa Y. Soyabean, goitre, and
prevention. J Tropical Med 1988; 34:110-113.
- Van Wyk JJ, Arnold MB, Wynn J, Pepper F. The effects of a soybean
product on thyroid function in humans. Pediatrics 1959; 24:752-760.
- Hydovitz JD. Occurrence of goiter in an infant on a soy diet.
N Engl J Med 1960;262:351–3.
- Shepard TH, Gordon EP, Kirschvink JF, McLean CM. Soybean goiter.
New Engl J Med 1960; 262:1099-1103.
- Ripp JW. Soybean-induced goiter. Am J Dis Child 102:106–109
(1961).
- Chorazy PA, Himelhoch S, Hopwood NJ, Greger NG, Postellon DC.
Persistent hypothyroidism in an infant receiving a soy formula:
case report and review of the literature. Pediatrics 1995;
96:148-50.
- Divi RL, Chang HC, Doerge DR. Anti-thyroid isoflavones from
soybean: isolation, characterization, and mechanisms of action.
Biochem Pharmacol 1997; 54:1087-96.
- Chang HC, Doerge DR. Dietary genistein inactivates rat thyroid
peroxidase in vivo without an apparent hypothyroid effect.
Toxicol Appl Pharmacol 2000; 168:244-52.
- Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity
of soy isoflavones. Environ Health Perspect 2002; 110 (Suppl
3):349-353.
- Ham JO, Chapman KM, Essex-Sorlie D, et al. Endocrinological
response to soy protein and fiber in midly hypercholesterolemic
men. Nutr Res 1993; 13:873-884.
- Duncan AM, Merz BE, Xu X, Nagel TC, Phipps WR, Kurzer MS.
Soy isoflavones exert modest hormonal effects in premenopausal
women. J Clin Endocrinol Metab 1999; 84:192-7.
- Duncan AM, Underhill KE, Xu X, Lavalleur J, Phipps WR, Kurzer
MS. Modest hormonal effects of soy isoflavones in postmenopausal
women [published erratum appears in J Clin Endocrinol Metab
2000 Jan; 85(1):448]. J Clin Endocrinol Metab 1999; 84:3479-84.
- Persky VW, Turyk ME, Wang L, et al.: Effect of soy protein
on endogenous hormones in postmenopausal women. Am J Clin
Nutr 2002;75:145–153.
- Bruce B, Messina M, Spiller GA. Isoflavone supplements do not
affect thyroid function in iodine-replete postmenopausal
women. J Med Food 2003; 6:309-316.
- Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests
in infants with congenital hypothyroidism: the influence
of soy-based formula. J Am Coll Nutr 1997; 16:280-2.
- Conrad SC, Chiu H, Silverman BL. Soy formula complicates management
of congenital hypothyroidism. Arch Dis Child 2004; 89:37-40.
- Bell DS, Ovalle F: Use of soy protein supplement and resultant
need for increased dose of levothyroxine. Endocr Pract 2001;
7:193–194.
- Pinchera A, MacGillivray H, Crawford JD, Freeman AG. Thyroid
refractiveness in an athyreotic cretin fed soybean formula.
N Engl J Med 1965; 273:83-87.
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