Thyroid Hormones (T3, T4 and TSH): What They Do, the Tests by Name and the Indian Studies

The thyroid is a small, butterfly-shaped gland at the front of the neck. It makes two hormones, thyroxine (T4) and triiodothyronine (T3), which control how the body uses energy and so affect nearly every organ, the heart included. A third hormone, TSH, comes from the pituitary gland and sets how hard the thyroid works. Too little thyroid hormone is hypothyroidism and too much is hyperthyroidism. In a study of 5,360 adults across eight Indian cities, 10.95 per cent had hypothyroidism. This entry in NNWA's handbook of hormones explains the hormones, the tests and the food questions, and marks what belongs to a doctor.

What are T3, T4 and TSH?

They are three links in one chain. T4 is the main hormone the thyroid releases. The joint FAO and WHO report on mineral requirements describes how cells take up T4 from the blood and remove one iodine atom to turn it into T3, the active form, which then binds to a receptor in the cell nucleus and switches genes on. TSH, thyroid stimulating hormone, is made in the pituitary gland and tells the thyroid how much T4 and T3 to make.

The same FAO and WHO report sorts the actions of thyroid hormone into two groups: growth and development, above all of the brain in the foetus and young child, and control of the body's metabolic processes.

How does the body keep thyroid hormone steady?

Through a feedback loop between the pituitary and the thyroid. When the thyroid makes too little hormone, the pituitary keeps releasing TSH to push it harder, so TSH in the blood rises. When there is too much thyroid hormone, the pituitary stops releasing TSH and the level falls. The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) gives this as the reason doctors usually check TSH first: it moves in the opposite direction to the thyroid's output.

Most T4 in the blood travels attached to carrier proteins and is held in reserve; only the unbound part, called free T4, can enter tissues. Pregnancy and oral contraceptive pills raise the amount of binding protein and so raise total T4 without any disease, while severe illness and steroid medicines can lower it. For this reason many doctors prefer to measure free T4. One practical note: a BMR calculator estimates resting energy from body measurements and cannot see thyroid status.

Which nutrients does the thyroid need from food?

Iodine first, then selenium. Iodine is part of the hormone itself: T4 carries four iodine atoms and T3 three, and the FAO and WHO report attributes every biological action of iodine to the thyroid hormones. Selenium works one step later. A 2009 review notes that the three deiodinase enzymes, which activate and inactivate thyroid hormone, are selenium-containing proteins. The handbook's iodine entry and selenium entry give the requirements in full.

Neither nutrient is a thyroid tonic. ICMR-NIN's 2024 Dietary Guidelines tell households to use iodised salt and keep added salt to 5 g a day, and for most people that settles the iodine question. NIDDK cautions that people with Hashimoto's disease or other autoimmune thyroid disorders may be sensitive to iodine's harmful effects, and that kelp, other seaweeds and iodine supplements may cause or worsen hypothyroidism.

Thyroid disorders in India: what the studies found

The national health survey asks people whether they have a thyroid disorder and does not test for one, so the picture comes mainly from city studies. The largest, published in 2013, tested 5,360 adults in Bangalore, Chennai, Delhi, Goa, Mumbai, Hyderabad, Ahmedabad and Kolkata. Hypothyroidism was found in 10.95 per cent: 7.48 per cent already knew and 3.47 per cent were newly detected. Women were affected about three times as often as men, and antibodies against the thyroid were present in 21.85 per cent.

The eight-city study was urban, recruited people who chose to attend one of 94 general health camps, and was funded by Abbott India Limited; its authors state that rural prevalence remains unknown. The Delhi study found that its high rate of subclinical hypothyroidism was not explained by iodine intake or by thyroid antibodies. NFHS-5 relies on what people report, which is why its figure is so much lower: in women it rose from 2 per cent in 2015-16 to 3 per cent in 2019-21.

Which thyroid tests might a doctor order?

Usually TSH first, then free T4, and sometimes T3 and thyroid antibodies. NIDDK states that a diagnosis cannot be based on symptoms alone, because tiredness, weight gain, feeling cold, dry skin, thinning hair, heavy or irregular periods and low mood have many other causes. The table names the tests you may see. What a given number means depends on the laboratory's reference range, age, pregnancy, medicines and the rest of the picture, so reading it is the doctor's work.

The last row matters more than its place suggests. A baby born with an underactive thyroid can suffer intellectual disability and poor growth if untreated, and early treatment prevents both. An ICMR task force analysed 104,006 heel-prick samples from five Indian centres to set normal values for newborns: 92.8 per cent had a TSH below 5 mIU per litre, and the group proposed 10 mIU per litre as the level above which a second sample should be sought.

A worked example: one report, read with care

This is an illustrative case, not a real person, and the numbers are chosen to show the reasoning, not to say what is normal. A 38-year-old schoolteacher in Kolkata, tired for some months and a little heavier, has a health check. The report shows TSH of 7.2 micro-units per millilitre and a free T4 of 1.1 ng per decilitre. A cousin says to stop soya and start kelp.

Do soya, cabbage and other goitrogens harm the thyroid?

In the amounts people eat, and with enough iodine, the trial evidence says no. A 2019 systematic review pooled 18 randomised trials of soya products and thyroid tests. Free T3 and free T4 did not change, and TSH rose by 0.248 mIU per litre, a shift the authors called very modest and of unclear clinical significance. The trials mainly gave supplements of soya isoflavones, extract or protein at 40 to 200 mg a day, not soya bean cooked as food.

Cabbage, cauliflower, broccoli and mustard greens are brassica vegetables, long suspected of a goitrogenic effect. A 2024 systematic review of 123 laboratory, animal and human studies concluded that most results cast doubt on an antithyroid effect in humans, and that brassica vegetables in the daily diet, particularly with adequate iodine intake, pose no adverse effect on thyroid function. The practical lists, food by food, are in NNWA's article on Indian foods and hypothyroidism, which this entry does not repeat.

What does the evidence say about selenium supplements?

It is mixed, and it concerns blood tests more than how people feel. A 2024 meta-analysis in the journal Thyroid combined 35 randomised trials in Hashimoto's thyroiditis. In people not taking thyroid hormone, selenium lowered TSH slightly (seven cohorts, 869 participants) and it lowered anti-TPO antibodies, while free T4, T3 and thyroid volume did not change; the authors rated the certainty as moderate.

None of this makes selenium a treatment to start alone. The gains were in laboratory values, and ordinary meals supply the 40 mcg a day ICMR-NIN advises: 60 g of whole wheat atta and 30 g of masoor dal together carry about 47 mcg in the Indian Food Composition Tables. Whether a supplement has any place beside prescribed treatment is a question for the treating doctor.

Levothyroxine and food: timing is the prescriber's call

Hypothyroidism is treated by replacing the missing hormone with levothyroxine, a medicine identical to the T4 a healthy thyroid makes. A doctor may advise taking it in the morning before eating, checks the blood test some weeks after starting, and adjusts the dose from the results. This entry gives no dose and no schedule. The amount and the timing are set for each person by the prescriber.

Food matters because it can block absorption. A 2021 systematic review of 63 studies found that coffee, soya products, fibre, and calcium or iron supplements each reduced levothyroxine absorption, though the evidence for each was limited, and that keeping a proper interval between the medicine and these items removes the interaction. If your breakfast, chai or supplements have changed, tell the prescriber; do not rearrange the tablet yourself.

Common myths and mistakes

A second mistake is to treat an underactive thyroid with iodine or kelp, which NIDDK warns may cause or worsen hypothyroidism in autoimmune thyroid disease. A third is to swap iodised salt for pink or rock salt and lose the household's main iodine source. A fourth is to blame every kilo on the thyroid: NIDDK points out that fatigue and weight gain are common and do not necessarily mean a thyroid problem. The guide to diet in hypothyroidism deals with eating alongside treatment.

Too much hormone, medicines and when to see a doctor

An overactive thyroid has its own signs: weight loss despite a good appetite, a rapid or irregular heartbeat, nervousness, shaky hands, sweating, heat intolerance and sometimes a swelling in the neck. Graves' disease is the commonest cause, and too much iodine or too much thyroid medicine can also produce it. NIDDK warns that taking more thyroid hormone than prescribed can cause serious problems such as atrial fibrillation and osteoporosis.

Nothing on this page is a reason to start, stop, skip or change a thyroid medicine. NIDDK's instruction is direct: never stop taking it without talking to your doctor first. Untreated hypothyroidism can affect both mother and baby, many women on thyroid medicine need a different dose once pregnant, and the advice is to contact the doctor as soon as a pregnancy is known. The pregnancy diet guide covers food in those months.

Learning this with NNWA

NNWA is a private skill academy based in Kolkata, with online teaching in English and in Hindi. Its courses train people to coach healthy clients on food and habits and to send medical questions to a doctor, and none is a degree or a dietitian qualification. The Thyroid Nutrition course is the short course on this subject.