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food source · 3
←Kelp
Kelp concentrates iodine from seawater to very high levels, and one serving can far exceed a day's need. It is a source of iodine and also one whose portion needs limiting: too much iodine disrupts the thyroid just as too little does.
Mechanism · Seaweed concentrates iodine
Iodine is dissolved in seawater, and large brown seaweeds such as kelp build it up in their tissues to a level almost unmatched among natural foods. The thyroid regulates how much iodine it takes in, but a large excess over a long time can still disrupt hormone production.
Safety · Use a little, and ask your doctor first
A pinch of dried kelp or a bowl of kelp soup can hold tens to hundreds of times the US adult recommended intake of 150 µg, and it is common for a single serving of commercial kelp or seaweed supplements to exceed the tolerable upper intake level, so kelp is best used in small amounts, for example to make stock. People with thyroid disease or who take thyroid medication should ask their doctor before eating a lot of kelp or taking seaweed supplements.
Seafood is generally rich in iodine, and shrimp is a source too. Iodine is the raw material the thyroid uses to make its hormones, and every thyroid hormone molecule carries several iodine atoms.
Mechanism · Iodine attaches to tyrosine
The thyroid uses a dedicated pump to pull iodine in from the blood against the concentration gradient, attaches it to tyrosine units on a large protein, and then joins two iodine-bearing tyrosines into a hormone. The numbers in T4 and T3 are the count of iodine atoms on the molecule. So iodine is not an add-on; it is part of the hormone molecule itself.
In practice · Enough, not more
Seafood is a reliable source of iodine, but reliable means enough, not the more the better: where salt is iodized, everyday intake is usually not short, and a large excess over a long time can also disrupt thyroid hormone production. The sodium in shrimp comes mostly from processing and dipping sauces, not from the shrimp itself.
Iodine is dissolved in seawater and passes up the food chain, so sea fish are one source of it. The amount varies with species and waters, so eating seafood regularly is a more practical way to cover iodine than counting on any one fish.
Mechanism · Iodine washed into the sea
Most of the earth's iodine has been washed into the sea by rain. Algae concentrate it from seawater and it passes up the food chain, so sea fish, kelp and nori contain iodine while freshwater fish have almost none. Iodine is a building block of the thyroid hormone molecule, and without it the hormone cannot be made.
In practice · Cover iodine by eating seafood often
The iodine in white fish varies with species and waters, and neither is fixed, so it is not worth counting fish by fish; the habit of eating seafood often is what covers it. Vitamin D is not white fish's strength either: raw cod has about 0.9 µg per 100 g against about 11 µg in salmon, a tenfold gap.
cofactor · 4
←Iron
The key enzyme that builds iodine into thyroid hormone, thyroid peroxidase (TPO), itself contains iron (as heme). So even with enough iodine, iron deficiency can leave thyroid hormone production short. In pregnancy, iron deficiency can drag the thyroid down with it, and too little thyroid hormone in pregnancy may affect the baby's brain development.
The deiodinases that convert thyroxine (T4) into active T3 are all selenium-containing proteins, so with too little selenium, tissue T3 can run short even when T4 is adequate. Selenium enzymes also clear the hydrogen peroxide produced as the thyroid makes hormone, protecting thyroid cells. Iodine is the raw material, selenium handles processing and protection, and both need to be adequate.
Iodine is the raw material of thyroid hormone: the thyroid builds it into T4 (four iodine atoms) and T3 (three). In other tissues a selenium-dependent deiodinase removes one iodine from T4 to make active T3. Too little iodine means too little hormone, and the pituitary pushes thyroid-stimulating hormone (TSH) up.
Iodine is the raw material for thyroid hormone. Without enough iodine, not enough hormone is made, the pituitary keeps releasing thyroid-stimulating hormone (TSH) to push the gland, and the thyroid follicles are stimulated to grow for years; this is one route to goiter and nodules (Zimmermann 2009).
regulates · 2
The thyroid needs iodine to make hormone, but people with Hashimoto's thyroiditis are more sensitive to iodine than others: long-term excess iodine is linked with worsening thyroid autoimmunity, while too little means too little hormone. The aim is enough without excess, not as much as possible; do not eat large amounts of high-iodine foods such as kelp every day.
←After birth and breastfeeding
The iodine in breast milk follows the mother's iodine intake, so getting enough matters while breastfeeding; the recommendation the US CDC cites is 290 micrograms a day. Cooking with iodized salt and eating seafood, eggs and milk supplies the raw material for the baby's thyroid.