I just got back from a conference on naturopathic endocrinology last weekend. One of the lectures focused on the effects of certain thyroid disrupting chemicals, what they do in the process of thyroid hormone production and utilization, and how to avoid them.

Quick Physiology Recap

The master endocrine gland in your brain called the hypothalamus is where it all starts. Your hypothalamus senses from the bloodstream how much of the active thyroid hormones T3 and T4 are already in circulation, and it changes its signal called TRH (Thyroid Releasing Hormone) to the pituitary gland according to what it perceives. (This is called a feedback mechanism.)

The pituitary (also in your brain, right underneath the hypothalamus) also picks up the levels of T3 and T4 in the bloodstream, as well as the TRH from the hypothalamus. According to these signals, it produces TSH (Thyroid Stimulating Hormone), which then travels to the thyroid itself.

The thyroid picks up the TSH signal, which tells it how much T4 (and small amount of T3) it ought to crank out. In order to do this, the thyroid concentrates iodine from the bloodstream (there’s about 50 times as much in the thyroid as there is in the blood), via NIS (the Sodium/Iodine Symporter). The Thyroid Peroxidase Enzyme (TPO) then takes iodide (a single molecule) and turns it into iodine so that it can be utilized in the formation of thyroid hormone.

Released thyroid hormone will then (mostly) need to be carried to peripheral tissues, via albumin, transthyretin (TTR), and thyrogloblin (Tg). Only a small percentage of thyroid hormone remains unbound.

Once T4 (most of it is this in the bloodstream) arrives at your peripheral tissues, it’s got to get converted into the more active T3 hormone.

Once you’ve got the T3 at the level of your tissues, it’s got to get transported into the nuclei of your cells to turn up the metabolism.

Finally, once your body is finished with the thyroid signal, it gets absorbed by your bile, and excreted out of your body.

Interference at the Hypothalamus/Pituitary

Chemicals that can block TRH docking from the hypothalamus to the pituitary include:

  • PCBs (Polychlorinated biphenyl). These are largely found in non-wild caught fish. Other sources include things you mostly can’t avoid unless you leave the Westernized world, including plasticizers, inks, adhesives, flame retardants, paints, wire insulators, elastic sealants, and heat insulators.

These patients may have a TSH that does not appear to respond to thyroid replacement—usually it appears abnormally low, despite low or normal T3 and T4.

Interference at the Thyroid

Chemicals that block TSH docking at the thyroid include:

  • PCBs (above).
  • DDT and DDE. These are pesticides, found in the Dirty Dozen non-organic fruits and veggies.

Chemicals that disrupt the iodine symporter (NIS) include the halides. I wrote here on the chemical group called halides that act as competitive inhibitors of iodine in the body, resulting in decreased concentration of iodine in the thyroid. These include:

  • Perchlorate. Sources of this include drinking water, chewing tobacco, and non-organic dairy.
  • Bromine, Chlorine and Fluorine (addressed in this article).
  • Mercury also falls under this category, found in farmed or Atlantic fish or dental amalgams. Another chemical that does this are phthalates, found in soft plastics and leeching into your food and water.

Chemicals that disrupt the Thyroid Peroxidase Enzyme (TPO) include:

  • Herbicides: found in the Dirty Dozen non-organic fruits and veggies.
  • Benzophenone: found in sunscreens. (Yep. Read your labels, folks.)

All of these mean the thyroid can’t keep up with the demand, so the pituitary has to shout louder, leading to the classic hypothyroid picture of high TSH and low T4 and T3.

Interference at the Carrier Molecules

Chemicals that interrupt the work of the carrier molecules include:

  • PCBs (above).
  • phthalates (above).
  • PBDE (Polybrominated diphenyl ethers). Used in fire retardants applied to foam furniture, carpets, and in a variety of other industrial products, these also don’t break down very well and therefore bioaccumulate. You can’t really avoid these.
  • pentachlorophenol (PCP): used as a pesticide and a wood preservative. Fortunately according to the EPA, it’s no longer used as a pesticide commercially.

This means even though all your labs look normal (TSH, T4, and T3), they’re not getting to your tissues and you’re still functionally hypothyroid.

Interference at the Peripheral Tissues

An enzyme called deiodinase is necessary to convert T4 to T3. Chemicals that block deiodinase activity include:

  • PCBs (above).
  • Mercury (above).
  • Triclosan. The most frequently used antibiotic in household products like soap.
  • pentachlorophenol. (above).

Another mechanism for this is that a certain percentage (usually about half) of T4 converts into reverse T3 (rT3), which is inactive. This is a way for the tissues to protect themselves against overstimulation, but mercury can increase the conversion of T3 to rT3.

This will look like adequate TSH and T4 levels, but relatively low T3.

Interference at the Cellular Transport Level

Chemicals that interfere with the body’s ability to import active T3 into the cell nucleus so it can do its job include:

  • PCBs (above).
  • BPA (Bisphenol A). Used in hard plastics and in the resin lining the insides of most canned foods. Even those that say ‘BPA-free’ are lined with another chemical in the bisphenol family. Best to avoid canned foods altogether.
  • PBDE (above).

This will look like adequate TSH, T3, and T4, but persistent hypothyroid symptoms.

Interference at the Excretion Level

Chemicals that cause increased excretion of thyroid hormones from the body include:

  • PBDE (above).
  • PCBs (above).
  • Dioxins. These are often byproducts of industrial processes involving chlorine like waste incineration, paper bleaching, and manufacturing chemicals and pesticides. (We can’t do much about our exposure, in other words.) Once released, it tends to concentrate in the food chain, particularly in dairy, meat, and fish.
  • Chlordane. Used as a pesticide, this is found primarily in the Dirty Dozen non-organic fruits and veggies.
  • Acetochlor. An herbicide, this is also found primarily in the Dirty Dozen non-organic fruits and veggies.

This, too, will look like adequate TSH, T3, and T4, but persistent hypothyroid symptoms.

Avoid Thyroid Toxins

Bottom line: if you want to avoid as many of the thyroid-disrupting chemicals as you can (without getting too obsessive—because that’s possible too), here are the keys:

  • Buy the Dirty Dozen fruits and veggies organic. Or if you can’t afford that, at least go for a really good fruit and vegetable wash.
  • Avoid farmed or Atlantic fish. (They’ll say this on the packaging.) Choose wild caught Alaskan, or Pacific.
  • Get a good filter for your water, and make sure the one you choose filters out the chemicals specific to where you live.
  • If you’re a dairy drinker, get it organic.
  • Avoid processed bread products (where the bromine shows up). Sprouted whole grains are generally a safer bet.
  • Read your sunscreen labels. In general, you want the ingredient list to be short and recognizable — zinc-based sunscreens are better choices, but at least make sure there’s no benzophenone in there.
  • Avoid soft plastics (i.e. none of those flimsy plastic water bottles; go for stainless steel or glass). Never never put seran wrap (or styrofoam) over your food and stick it in the microwave. Store your leftovers in glass if you can, but if you must use plastic, at least don’t put it in the microwave to heat it up before you eat it!
  • Don’t pick hand soaps that contain triclosan.
  • Avoid canned foods. Fresh is always best of course; but if you can’t get fresh, choose frozen or dried instead.
  • Buy your meat products free range or organic. If you can’t afford this, focus instead on plant-based proteins.

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