Re: genistein and daidzein

 Inactivation of thyroid peroxidase by genistein and daidzein in vitro and in vivo:
mechanism for anti-thyroid activity of soy.

This is an animal study done on rats, not humans.  And they were exposed to high doses for their body size, a typical tactic used to "prove" something is dangeous. Human studies show a different story:

http://www.ncbi.nlm.nih.gov/pubmed/18624607?ordinalpos=4&itool=EntrezSyst...

Short-term effect of soy consumption on thyroid hormone levels and correlation with phytoestrogen level in healthy subjects.

OBJECTIVE: Since soy isoflavones may influence the thyroid hormone feedback system by interference with their biosynthesis, secretion and metabolism, we tested whether their controlled shortterm consumption affects thyroid function. METHODS: Eighty six volunteers--university students (32 males and 54 females) were eating unprocessed boiled natural soybeans (2 g/kg body weight/day) for 7 consecutive days. Thyrotropin, free thyroid hormones, antibodies to thyroid peroxidase and to thyroglobulin, and actual levels of unconjugated major soy phytoestrogens, daidzein and genistein, were measured in sera collected before, at the end and one week after finishing soy meal consumption. RESULTS: Both phytoestrogens increased significantly (p<0.0001) at the end of soy-diet and fell down after its termination nearly back to the initial values. No significant changes were found in female group, while in males a significant transitory increase of thyrotropin (p<0.0001) was recorded. When actual levels of phytoestrogens were related to thyroid parameters, the only significant correlations were found between basal levels of daidzein and thyrotropin, daidzein and antithyroglobulin at the end of soy consumption in males, and between daidzein and free thyroxine at the end of the soy ingestion in females. CONCLUSION: Though only modest and transitory effects on thyroid parameters occurred after controlled short-term soy consumption, some actual thyroid hormone parameters do correlate with actual isoflavone levels.

 

http://www.ncbi.nlm.nih.gov/pubmed/17316115?ordinalpos=1&itool=EntrezSyst...

Soy protein isolates of varied isoflavone content do not influence serum thyroid hormones in healthy young men.

OBJECTIVE: The ability of soy isoflavones to inhibit thyroid peroxidase and induce goiter in animals has generated concern regarding their potential antithyroid effects in humans. The purpose of this study was to determine the effects of soy protein isolates of varied isoflavone content on circulating thyroid hormones in healthy young men. DESIGN: Thirty-five healthy men (27.9 +/- 5.7 years old) supplemented their habitual diets with milk protein isolate (MPI), low-isoflavone soy protein isolate (low-iso SPI; 1.64 +/- 0.19 mg iso/day), and high-isoflavone SPI (high-iso SPI; 61.7 +/- 7.4 mg iso/day) for 57 days each, separated by 4-week washouts in a randomized crossover design. Blood was collected on days 1, 29, and 57 of each treatment for analysis of total triiodothyronine (T3), free T3, total thyroxine (T4), free T4, thyroid stimulating hormone (TSH), and thyroid binding globulin (TBG). Twenty-four hour urines were collected at the end of each treatment for analysis of isoflavones. MAIN OUTCOME: Results revealed no significant effects of the low-iso or high-iso SPIs on serum total T3, free T3, total T4, free T4, TSH, or TBG when compared with the MPI on either study days 29 or 57. Urinary data revealed that isoflavones were significantly increased by the high-iso SPI relative to the low-iso SPI and MPI. CONCLUSIONS: Results of this study demonstrate that soy isoflavones in a protein matrix do not significantly influence circulating thyroid hormones in healthy young men.

 

http://www.ncbi.nlm.nih.gov/pubmed/17472472?ordinalpos=1&itool=EntrezSyst...

Seaweed and soy: companion foods in Asian cuisine and their effects on thyroid function in American women.

Seaweeds and soy are two commonly eaten foods in Asia. Both have been reported to affect thyroid function, seaweed because of its iodine content and soy because of its goitrogenic effect. Twenty-five healthy postmenopausal women (mean age 58 years) completed a double-blinded randomized crossover study. Ten capsules (5 g/day) of placebo or seaweed (Alaria esculenta), providing 475 microg of iodine/day, were consumed daily for 7 weeks. A powdered soy protein isolate (Solae Co., St. Louis, MO), providing 2 mg of isoflavones/kg of body weight, was given daily during the last week of each treatment arm. On average, this provided 141.3 mg of isoflavones/day and 67.5 g of protein/day. Blood samples and 48-hour urine samples were collected before and after each intervention period, and urinary I/C (microg of iodine/g of creatinine) and serum thyroxine, free thyroxine index, total triiodothyronine, and thyroid stimulating hormone (TSH) were measured. Seaweed ingestion increased I/C concentrations (P < .0001) and serum TSH (P < .0001) (1.69 +/- 0.22 vs. 2.19 +/- 0.22 microU/mL, mean +/- SE). Soy supplementation did not affect thyroid end points. Seven weeks of 5 g/day seaweed supplementation was associated with a small but statistically significant increase in TSH. Soy protein isolate supplementation was not associated with changes in serum thyroid hormone concentrations.

 

http://www.ncbi.nlm.nih.gov/pubmed/14977438?ordinalpos=1&itool=EntrezSyst...

Isoflavone supplements do not affect thyroid function in iodine-replete postmenopausal women.

Despite the safety review conducted by the U.S. Food and Drug Administration in the process of awarding a health claim for the cholesterol-lowering properties of soy protein, concerns about the possible goitrogenic effects of soybean isoflavones persist. Concerns are based primarily on in vitro research, animal studies, and older reports of goiter in infants fed soy formula not fortified with iodine. In a randomized, double blind, placebo-controlled study, we investigated the effect on thyroid function of a daily supplement containing 90 mg (aglycone weight) of total isoflavones/day versus placebo in 38 postmenopausal women, 64-83 years old, not on hormone replacement therapy. Serum thyroid-stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) were measured at baseline and after 90 and 180 days. In the supplement group, at baseline and 6 months, TSH (micro U/ml), T4 (nM), and T3 (nM) levels (mean +/- SE) were 3.00 +/- 0.44, 149.00 +/- 5.04, and 1.53 +/- 0.13, respectively, and 3.49 +/- 0.52, 154.52 +/- 2.09, and 1.78 +/- 0.12, respectively. In the control group, levels at baseline and at 6 months were 3.35 +/- 0.51, 145.39 +/- 6.69, and 1.55 +/- 0.18, respectively, and 3.63 +/- 0.57, 153.77 +/- 6.64, and 1.75 +/- 0.10, respectively. Intragroup differences for all three measures were statistically indistinguishable at 6 months, and levels were similar between the isoflavone supplement and placebo groups at each measurement. These results indicate that in this group of healthy iodine-replete subjects, soy isoflavones do not adversely affect thyroid function.

 

http://www.ncbi.nlm.nih.gov/pubmed/16475902?ordinalpos=14&itool=EntrezSys...

Actual levels of soy phytoestrogens in children correlate with thyroid laboratory parameters.

Thyroid hormones and thyroid autoantibodies, along with serum concentrations of two phytoestrogens of the isoflavone series, daidzein and genistein, were measured in 268 children without overt thyroid diseases, screened for iodine deficiency in one region of the Czech Republic. Since both phytoestrogens have been reported to inhibit thyroid hormone biosynthesis and in high concentrations to exert goitrogenic effects, we investigated whether their presence in the circulation could influence thyroid hormone function in a population where soy consumption is not common. Correlation analysis revealed a significant positive association of genistein with thyroglobulin autoantibodies and a negative correlation with thyroid volume. Multiple regression analysis of the relationships between actual phytoestrogen levels and measured thyroid parameters revealed only a weak but significant association between genistein and thyroid variables. Higher levels of free thyroxine were found in a subgroup of 36 children who ate soy food in the previous 24 h. In conclusion, only modest association was found between actual phytoestrogen levels and parameters of thyroid function. On the other hand, even small differences in soy phytoestrogen intake may influence thyroid function, which could be important when iodine intake is insufficient.

 

The studies provide another possible answer to why some people have problems and some don't. Iodine levels.

 

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