What the Thyroid Actually Does
Your thyroid is a butterfly-shaped gland at the front of your neck. It produces hormones that set the pace of your metabolism — how fast your cells burn energy, how your body uses carbohydrates and fat, how you regulate temperature, and how alert or calm you feel.
When thyroid function drops, your metabolism slows. When it rises, everything accelerates. The effects are systemic: thyroid hormones influence heart rate, digestive speed, body temperature, mood, and how efficiently you convert food into usable energy.
How the System Works
Your thyroid doesn't act on its own. It's controlled by a feedback loop that runs through your brain — and understanding this loop explains why doctors check TSH on blood tests, why selenium matters for hormone activation, and why crash diets slow your metabolism.
Here's the chain:
Your hypothalamus (a brain region) monitors thyroid hormone levels in your blood. When they drop, it sends a signal to the pituitary.
Your pituitary gland responds by releasing TSH — thyroid-stimulating hormone. TSH is the direct order to your thyroid: make more hormone.
Your thyroid responds, producing mainly T4 (thyroxine) along with some T3 (triiodothyronine).
When T4 and T3 levels rise back up, the hypothalamus and pituitary dial down their signals. When levels fall, they ramp up. Because TSH rises before thyroid hormone levels drop below normal, it's the most sensitive early indicator of a thyroid problem — which is why doctors check TSH first when screening for thyroid issues.
T4 to T3: The Conversion Step That Nutrition Directly Affects
The thyroid produces about 80–90% T4 and only 10–20% T3. But T4 is mostly a storage form — your cells can't use it directly. Your body needs to convert T4 into T3 by stripping off one iodine atom. This conversion happens mainly in the liver, kidneys, and muscle, carried out by enzymes called deiodinases.
Selenium is built into the structure of these deiodinase enzymes — it's not just "supportive," it's part of the hardware. Without enough selenium, the conversion from T4 to T3 slows, and you can end up with normal T4 levels but insufficient active T3 reaching your cells. This pattern, sometimes called poor conversion, doesn't always show up on standard blood tests.
Even if your thyroid is producing enough hormone, your body may not be activating enough of it if selenium is deficient.
The Nutrients That Matter
Iodine: The Raw Material
Iodine is the single element your body cannot make thyroid hormones without. Each molecule of T4 contains four iodine atoms; T3 contains three. An enzyme in the thyroid gland — thyroid peroxidase — attaches iodine to a protein scaffold to build these hormones. Without adequate iodine, production drops, TSH rises, and the thyroid can enlarge trying to capture more iodine from the blood, a condition called goitre.
In most developed countries, iodine deficiency is uncommon because salt is iodised — a public health intervention that has been remarkably effective. If you use regular iodised salt in cooking, you're likely covered.
If you primarily use sea salt, kosher salt, or avoid added salt, your iodine intake may be lower. Food sources that reliably contain iodine:
Fish and seafood (cod, tuna, prawns — roughly 50–100 µg per 100 g)
Seaweed (variable but often very high — nori, wakame, kombu)
Dairy products (milk and yoghurt — roughly 50–100 µg per 250 ml of milk)
Eggs (roughly 25 µg per egg)
Adults need about 150 µg of iodine per day. That's not a lot, but the margin between adequate and insufficient is narrow if you're not using iodised salt.
Too much iodine is also harmful. Excess iodine can trigger both hyper- and hypothyroidism, particularly in people with underlying thyroid disease. This is rare from food alone, but kelp supplements and concentrated seaweed products can deliver doses well above the safe upper limit of 1,000 µg per day. More is not better.
Selenium: The Conversion Enzyme
Selenium is structurally embedded in the deiodinase enzymes that convert T4 to T3. It also forms part of an antioxidant enzyme that protects thyroid tissue from the oxidative stress generated during hormone production.
Adults need about 55 µg per day. Food sources:
Brazil nuts — roughly 65–90 µg per nut (2–3 nuts exceed the daily target)
Fish and seafood — tuna, sardines, prawns (roughly 30–50 µg per 100 g)
Meat and poultry — chicken, turkey, pork (roughly 15–30 µg per 100 g)
Eggs — roughly 15–20 µg per egg
Whole grains and legumes — amounts vary with soil selenium content
Brazil nuts are the most concentrated source, but the selenium content per nut varies depending on the soil they were grown in. Two to three nuts per day is a reasonable pattern; eating a handful daily can push intake above the safe upper limit of 400 µg, which over time can cause selenosis — hair loss, nail brittleness, gastrointestinal issues.
Iron and Zinc: Supporting Roles
Iron is needed for the same enzyme that attaches iodine to build thyroid hormones — iron deficiency impairs this step directly. Zinc supports the deiodinase enzymes alongside selenium and also plays a role in the brain signalling that regulates TSH release.
These aren't thyroid-specific nutrients, but deficiency in either can contribute to low thyroid function independently of iodine and selenium status. Iron deficiency is one of the most common nutrient deficiencies worldwide, and it's worth being aware of if you eat little or no red meat.
Patterns That Directly Affect Thyroid Function
Severe Calorie Restriction Suppresses Thyroid Hormone
When you dramatically cut calories, your body doesn't just burn less fat — it downregulates thyroid hormone production to conserve energy. T4-to-T3 conversion drops, and reverse T3 (an inactive form) increases. This isn't a malfunction; it's an evolutionary adaptation to famine. But if you're restricting calories to lose weight, the resulting metabolic slowdown works against you.
Eating enough — particularly enough carbohydrate — signals that resources are available, and the feedback loop operates normally. The threshold varies by person, but prolonged intake below roughly 1,200 kcal per day is where suppression becomes likely.
Very Low-Carb Diets Impair T4→T3 Conversion
The T4-to-T3 conversion is carbohydrate-sensitive. Very low-carbohydrate diets (under roughly 50 g per day) can reduce T3 levels and increase reverse T3, effectively slowing the metabolic rate. This doesn't mean you need a high-carb diet — it means including some whole grains, legumes, starchy vegetables, or fruit supports the conversion pathway. If you're on a strict ketogenic diet and experiencing fatigue or cold intolerance, carbohydrate intake may be part of the picture.
Goitrogens: A Real Mechanism, an Overblown Risk
Cruciferous vegetables — broccoli, cabbage, kale, cauliflower, Brussels sprouts — contain compounds called goitrogens that can interfere with the enzyme that builds thyroid hormones, particularly when iodine intake is already low.
But the practical risk is minimal for three reasons:
Goitrogenic effects are mainly observed in iodine-deficient populations
Cooking substantially reduces goitrogen content
You would need to eat very large quantities of raw cruciferous vegetables daily to produce a measurable effect
Avoiding broccoli and kale for thyroid health is a bad trade — these vegetables deliver fibre, antioxidants, and nutrients that matter far more than the trace goitrogen content. If your iodine intake is adequate, goitrogens from normal portions of cooked cruciferous vegetables are not a credible threat.
Soy: Interferes at High Doses, Not at Normal Intake
Soy isoflavones can inhibit the enzyme that builds thyroid hormones in lab studies, and some animal studies show effects at high doses. Human data is more mixed: a meta-analysis found no significant effect of soy on thyroid function in adults with adequate iodine and normal thyroid function. People with existing hypothyroidism may need somewhat higher levothyroxine doses if they consume soy regularly.
Normal intake of tofu, tempeh, soy milk, or edamame does not cause thyroid problems. The relevant precaution: if you have diagnosed hypothyroidism and take levothyroxine, soy can interfere with medication absorption — separate soy consumption from your medication by several hours.
When Nutrition Isn't Enough
Hashimoto's thyroiditis (autoimmune hypothyroidism) is the most common cause of low thyroid function in iodine-sufficient countries. Graves' disease (autoimmune hyperthyroidism) is the most common cause of overactive thyroid. Both require medical management. Nutrition supports these conditions but does not treat them.
Key considerations for people on thyroid medication:
Levothyroxine (synthetic T4) should be taken on an empty stomach, at least 30–60 minutes before eating
Calcium and iron supplements can block levothyroxine absorption if taken within 4 hours
Consistent soy intake may require a dose adjustment — not because soy is harmful, but because it changes absorption
If you have a diagnosed thyroid condition, the most impactful nutritional step is ensuring your medication is absorbed correctly. The nutrient patterns above still apply, but medication management comes first.
Pregnancy: Iodine Demand Increases Sharply
Iodine requirements rise to about 250 µg per day during pregnancy — substantially above the 150 µg adult baseline — because the fetus depends entirely on maternal thyroid hormone for brain development in the first trimester. Many prenatal supplements contain iodine, but not all do. If you're pregnant or planning pregnancy and don't regularly eat iodine-rich foods, verify that your supplement covers this gap.
The Compact Takeaway
Iodine and selenium are the two nutrients most directly tied to thyroid function — iodine for hormone synthesis, selenium for activation and protection
Use iodised salt or eat seafood, eggs, and dairy regularly; a few Brazil nuts per week covers selenium
Severe calorie restriction and very low-carb diets suppress thyroid hormone — eat enough, and include some carbohydrate
Goitrogens and soy are not practical concerns at normal intake levels with adequate iodine
If you have a thyroid condition, medication absorption matters more than any single food choice