I was sorry to read over at Megan McArdle’s blog over at the Atlantic yesterday that she has Hashimoto’s thyroiditis and as a consequence has to give up being a vegan. (Her diet was high in soy for protein, and there is some evidence that soy interferes with thyroid function. More on this in a bit.) Anyway, I noticed in the comments that there are lots of people that hadn’t ever heard of Hashimoto’s (and my Mom actually had it too), so I would say a couple of things to clarify.
Hashimoto’s thyroiditis is the most common kind of thyroid failure in areas of the world where people get enough iodine in their diet. It is a chronic autoimmune disease caused by the progressive destruction of the thyroid gland by antibodies and immune cells. Thyroid hormone — for those who don’t know — is important for regulating your metabolism. If you don’t have enough of it (hypothyroidism), you feel really dreary, gain weight, and feel cold all the time. If you have too much (hyperthyroidism), you eat gobs and gobs and you feel like you are having hot-flashes.
When it comes to prevalence, the core demographic is mostly older women. (It has about a 7:1 female gender bias, and your risk of hypothyroidism increases with age.) It almost certainly has genetic risk factors. We know this because people with this tend to have other autoimmune diseases such as type I diabetes or celiac disease. It also tends to run in families.
The trouble with Hashimoto’s is that while as an endpoint you end up with less thyroid function (the thyroid eventually dies), it can be a bit of a rollercoaster getting there. This is because thyroid hormone precursors are stored in the thyroid gland in what are called follicles (some nice pictures here). As the thyroid gland becomes inflamed, these follicles dissolve and are released into the blood at levels much higher than physiological. These high levels of thyroid hormone can cause what is called a thyroid storm — where you have heart palpitations and feel really feverish. Eventually as the follicles become depleted, you will (usually) go back to hypothyroid hormone levels. This up and down makes patients absolutely miserable. Fortunately, it is usually time limited.
With respect to treatment, when hypothyroidism is the problem, we usually use thyroid hormone replacement therapy. I’m sad to say that most of the time when you start this hormone replacement, it is life long. There are some examples where individuals with Hashimoto’s have recovered thyroid function, but they are in the minority. In most cases, the end result of Hashimoto’s is that you have to take thyroid hormone in pill form for the rest of your life. Fortunately, after a period of tapering, you can usually get by on the same dose, and thyroid hormone formulations are available as generics.
As an aside, I am ignoring here the details of thyroid regulation and metabolism. Thyroid hormone comes in two forms: a less active form called T4 and an active form called T3. T4 is converted to T3 in peripheral tissues by an enzyme called 5′-iodinase. Thyroid hormone release is triggered by another hormone released by the brain called Thyroid Stimulating Hormone or TSH. There is a feedback between T4 and T3 and TSH release — T4/T3 go up, TSH goes down and vice versa. Thus, the clinical picture in terms of blood tests for hypothyroid is low T4/T3, high TSH.
So, what is this deal with soy?
Well, we know that in animal models soy diet can produce a goiter. A goiter happens when you have too much TSH. TSH stimulates both T4/T3 release and growth of the thyroid gland cells. Thus, a goiter is a large growth of thyroid cells caused by the TSH being ineffective at raising T4/T3 — which would then lower the TSH. (You can also get a goiter from insufficient iodine in your diet. Iodine is necessary for thyroid hormone synthesis.) This is evidence in animals that suggests that individuals who consume large amounts of soy also need to consume extra iodine.
Likewise, there is some in vitro evidence that compounds found in soy called isoflavones can inhibit an enzyme called thyroid peroxidase — the enzyme that liberates T3 and T4 from the follicles. Thus, there is some convergence of evidence in vitro and in vivo that soy can inhibit proper thyroid function.
What is the clinical evidence for this effect though? Well, studies have looked at this issue are pretty spare, but actually very few of them show any effect of soy in people who do not have some underlying thyroid problem already. (The subject is reviewed here.) However, there is some evidence that in individuals who require thyroid supplementation, soy can interfere with the drug. This is probably because the soy is binding up the artificial thyroid hormone in the intestines and preventing its absorption. This is particularly true in hypothyroid infants who are giving soy milk. Thus, in people who have to take thyroid hormone (the most common types are Cytomel and Synthroid) it is probably not advisable to eat a lot of soy. (It may be possible to still include it in your diet, however. You would need to increase your dose and/or vary the time in the day that you take the drug. But this is something that you would definitely want to discuss with your endocrinologist.)
Anyway, that is all I have on Hashimoto’s and soy. I am sorry to hear about Megan’s condition, particularly as it is interfering with what is a very healthy life choice (ethical issues aside). My suspicion is that many of you know someone with Hashimoto’s, or you will at some time in your life. Now you can show off your knowledge. If you have questions, put them in the comments and I will try and look them up.