More people are eating soy products these days. Some eat soy because they avoid all animal products. Vegans know that soy is an excellent source of protein.
Some post-menopausal women are eating soy to help their bones.
But recent long term, randomized, controlled clinical trials do not show bone benefit from soy.
And, in most trials, hot flashes do not improve with soy.
And the earlier trials with soy also failed to show bone benefit from soy.
Although a recent study suggested that there might be a slight positive effect on bone strength.
Many people who eat soy are concerned that high soy intake can affect their thyroid.
So what do the controlled, clinical trials show with respect to soy and thyroid?
One of those trials looking at bone effects of soy in postmenopausal women showed no effects of soy on thyroid hormone levels.
In most men or women who have normal thyroid function to begin with, soy has no effect on thyroid activity. Trials in women were published in 2003, 2007, and 2011.
The exception regarding soy and thyroid: in those who are deficient in iodide or who have a failing thyroid, high soy intake is three times more likely to lower thyroid hormone levels further. The mechanism of this effect has been studied.
So high soy intake could make the thyroid more underactive if the thyroid is struggling to make enough hormone to begin with.
Iodide deficiency is simply not an issue in most developed countries.
See my previous posts on thyroid.
A failing thyroid is most commonly caused by the autoimmune condition called Hashimoto’s thyroiditis. Underactive thyroid is called hypothyroidism. Borderline hypothyroidism can be considered to be a failing thyroid and is usually called subclinical hypothyroidism.
So besides Hashimoto' disease, who else is more likely to have subclinical hypothyroidism?
Those who have only part of their thyroid because of surgery.
And those who have been treated with radioactive iodide for an overactive thyroid.
Both of these groups of people should be getting regular blood tests to monitor for hypothyroidism.
The best test to follow for those with hypothyroidism is the TSH.
TSH stands for thyroid stimulating hormone.
TSH is increased in those with inadequate thyroid hormone levels.
As the hormone production by the thyroid gland goes down, the TSH from the pituitary gland goes up. Check this out to learn more.
If you are already taking thyroid hormone for hypothyroidism it's no big deal if you eat soy.
Your TSH level ought to checked regularly by your doctor.
Your thyroid dosage should be adjusted according to those blood tests.
So be reassured that soy intake should not cause thyroid problems unless you are more likely to have untreated hypothyroidism to begin with.
If you have not had thyroid surgery and never was treated for overactive thyroid and do not have Hashimoto's disease, then eating lots of soy should cause no problems.
And if you are taking thyroid hormone for hypothyroidism, again no problem.
Ask your doctor if you are concerned. Your TSH level can be checked.
In general, if your TSH level is below 3, you are okay.
Above 3, generally means your thyroid hormone levels are not quite enough.
Go ahead and enjoy your soy in any case.
I hope this update helps those soy consumers out there.
Now please excuse me while I prepare my tofu stir-fry.
Daniel Weiss MD CDE FACP PNS CPI