A group of scientists from the UK (Sathaypalan. 2011) investigated the effect an 8 weeks dietary intervention with "high-dose phytoestrogens (30 g soy protein with 16mg phytoestrogens, representative of a vegetarian diet)" on thyroid function in sixty patients with subclinical hypothyroidism. The results were unequivocal - high dose phytoestrogens put your thyroid over the edge:
Six patients (10%) developed overt hypothyroidism after high-dose phytoestrogen, and none after low-dose phytoestrogen supplementation. [...] All six subjects were diagnosed as having overt hyothyroidism during their end-of-study visit after the 8-wk high-dose phytoestrogen supplementation period, when they were found to have raised TSH of more than 10 mU/liter and a low fT4 of less than 9 pmol/liter, i.e. overt hypothyroidism.Even in view of the fact that some "health markers" such as insulin resistance, hsCRP, and blood pressure improved in the 16mg (high dose) phytoestrogen group, the question remains, whether lifelong levothyroxin medication and the possible subsequent damage due to hypothyrodism (even with "adequate" medication, cf. stopthethyroidmadness.com) is worth it!?