Thyroid hormone sets how fast every cell burns fuel and makes heat. Cold hands, low energy, constipation and thinning hair are the classic signs it's running low. Making it takes iodine and iron. Activating it takes selenium and enough food. Stress and dieting turn it down.
The brain releases TSH, which tells the thyroid to grab iodine from the blood and attach it to tyrosine, an amino acid. That makes T4, the storage form. T4 travels to tissues, where deiodinase enzymes trim one iodine off to make T3, the active form, or a different one to make inactive reverse T3. T3 then switches on the genes that run mitochondria and heat production. Most problems people feel come from too little raw material, poor conversion, or the body deliberately turning T3 down because it thinks food is scarce.
The gland puts iodine onto tyrosine and joins two of them into T4, which carries four iodines. The enzyme that does it (TPO) is iron-based.
Selenium-based enzymes turn T4 into active T3. When food is short or cortisol is high, more goes to inactive reverse T3 instead.
T3 tells cells to build more mitochondria and burn more fuel. Brown fat turns it into heat when you're cold.
A normal TSH can still sit on top of poor conversion. That's why free T3 is worth measuring too.
Iodine deficiency is the world's most common cause of thyroid disease and of preventable brain damage in children. Iodized salt fixed it in the US in the 1920s. The modern twist: people who cook at home and switch to sea salt, kosher salt or pink salt, or who cut dairy, can slide back toward low iodine, because those salts carry almost none. US iodine intake roughly halved between the early 1970s and the early 1990s. Pregnant women need more, and a shortfall then affects the baby's brain.
Salt the way you like, and pay attention to the source. Iodized table salt, seafood, dairy and eggs are the reliable sources. Raw cruciferous vegetables and soy only matter much when iodine is already low.
Read more: Sodium: how much salt · Wild Fish & Oysters
↑ back to the short listThe body reads a calorie shortfall as famine and turns metabolism down to save fuel. T3 falls within a day or two of fasting, and reverse T3 rises. Long-term calorie restriction, very low-carb diets and hard training on little food all do the same. That's adaptive, not a disease, and TSH often stays normal. But it explains cold hands, low energy and stalled fat loss in chronic dieters. Insulin and carbohydrate help keep T4-to-T3 conversion going.
Read more: Metabolic Health · Stress: eat enough · GLP-1
↑ back to the short listGram for gram, the thyroid holds more selenium than any other organ. The deiodinases that make T3 are selenium-based enzymes, and selenium-based glutathione peroxidase protects the gland from the hydrogen peroxide it uses to make hormone. In Hashimoto's, a 2024 meta-analysis of 35 trials found selenium lowered thyroid antibodies, and lowered TSH in people not yet on medication. More isn't better. Selenium has a U-shaped curve, and high intakes raise diabetes risk.
Read more: Selenium: thyroid · the U-curve
↑ back to the short listThyroid peroxidase, the enzyme that attaches iodine to tyrosine, is iron-based. Iron deficiency slows hormone production, and in iodine-deficient children, iodine worked better once their iron was fixed. Low iron and low thyroid also cause the same symptoms: tired, cold, hair shedding. Women who menstruate are most often low. Check ferritin, not just haemoglobin, because stores run out before anaemia shows.
Read more: Iron
↑ back to the short listCortisol talks to the thyroid. High cortisol, whether from chronic stress, illness, short sleep or steroid drugs, blunts TSH release and pushes T4 toward reverse T3. Very sick hospital patients show low T3 with normal TSH for the same reason. Fixing sleep, eating enough and salting food lowers the background stress signal that keeps thyroid output down.
Cold tolerance is trainable. Brown fat burns fuel straight into heat, and cold switches on a deiodinase inside it that makes T3 locally, right where it's needed. In Dutch studies, 10 days of living at about 59–61 °F (15–16 °C) for six hours a day raised brown fat activity, and people shivered less and felt more comfortable. Regular cold water does the same faster. If you're cold all the time no matter what, though, think thyroid, iron or under-eating before you blame low brown fat.
Read more: Cold Exposure & Metabolic Health
↑ back to the short listInside the cell, T3 works by binding a receptor that pairs with a vitamin A receptor and grips DNA with zinc fingers. Low vitamin A raises TSH, and in a trial in children short on iodine and vitamin A, adding vitamin A lowered TSH and goitre size. Zinc deficiency lowers T3. Both are easy to cover with animal foods.
This is a quick guide. The evidence and references live on the linked pages. Not medical advice.