It weighs only about 20 grams, yet it helps decide how fast your heart beats, how warm you feel and how much energy you still have in the afternoon.
Most people only start thinking about their thyroid when something is wrong. Yet it pays to understand it beforehand.
What is the thyroid?
The thyroid is a hormone gland in the neck. It sits below the larynx and lies against the front of the windpipe.
Its shape is reminiscent of a butterfly: two lobes to the left and right of the windpipe, connected by a narrow bridge.
Its Latin name is glandula thyreoidea, named after the shield-shaped cartilage of the larynx in front of which it lies.
What does the thyroid do? (Short version)
The thyroid produces two hormones, thyroxine and triiodothyronine, T4 and T3 for short. These hormones set the pace of metabolism in practically every cell of your body.
Specifically, they influence:
- Energy turnover and body temperature. How much energy your cells use at rest.
- Heart and circulation. Heart rate and the strength of the heartbeat.
- Digestion. How fast the gut moves.
- Nervous system and concentration. Responsiveness, mood, mental alertness.
- Growth and development. Especially brain development before and after birth.
- Skin, hair and bones. Cell renewal and bone remodelling.
In addition, specialised cells in the thyroid, the so-called C cells, produce a third hormone: calcitonin. It is involved in calcium balance and therefore in bone metabolism.
How does the thyroid make its hormones?
The process in four steps:
- Take up iodine. The thyroid takes iodine from the blood and concentrates it strongly. The body cannot make iodine itself, it has to come from the diet.
- Provide the framework. It produces a large protein called thyroglobulin. It contains the amino acid tyrosine, the framework of the later hormones.
- Add the iodine. An enzyme called thyroid peroxidase attaches the iodine atoms to the tyrosine. Depending on the number, this gives T4 with four iodine atoms or T3 with three.
- Build up a store. The finished hormone is stored in the follicles (small vesicles in the tissue). This makes the thyroid the only hormone gland that builds up a real store. It lasts for several weeks to months.
T4 and T3: where the difference lies
The thyroid mainly releases T4, roughly 80 to 90 percent of its output.
T4, however, is hardly active. It is the storage and transport form. The actual work is done by T3, and most of it is only formed outside the thyroid: in the liver, kidneys, muscles and other tissues, one iodine atom is removed from T4, leaving T3.
It is an elegant system. The body can decide locally how much active hormone a tissue needs at a given moment, instead of controlling everything centrally.
The fact that natural preparations made from thyroid tissue contain both hormones, while synthetic levothyroxine contains only T4, is the reason for a decades-old debate among experts. What natural thyroid extract is all about is covered in a separate article.
How your body controls the amount
The thyroid does not work on its own. It is the lower end of a chain of three stations.
TRH is produced in the hypothalamus, an area in the brain. This signal goes to the pituitary gland, which then releases TSH. TSH in turn reaches the thyroid via the blood and prompts it to produce hormones.
The feedback is what matters. If the amount of T4 and T3 in the blood rises, the hypothalamus and pituitary turn their signals down. If it falls, they turn them up again.
This leads to a peculiarity that surprises many people: a high TSH level points to an underactive thyroid. The pituitary calls louder because too little is arriving. Conversely, a low TSH level points to an overactive thyroid.
That is why TSH is the first value a doctor measures when a thyroid problem is suspected.
How to tell that your thyroid is working normally
A healthy thyroid goes unnoticed. That is exactly its hallmark.
You are not constantly colder than others and you do not sweat for no reason. Your weight stays roughly stable with the same food and the same amount of exercise. Your resting pulse is in the normal range. You fall asleep and wake up reasonably rested. The front of your neck looks symmetrical, without visible swelling.
None of this is proof. All of these points also depend on sleep, stress, exercise and other factors. Only a blood test gives real certainty.
Frequently asked questions
What is the thyroid?
A butterfly-shaped hormone gland in the neck, below the larynx in front of the windpipe. In adults it weighs about 20 grams.
What does the thyroid do in the body?
It produces the hormones T4 and T3, which set the pace of metabolism in almost all body cells. This affects energy turnover, body temperature, heart rate, digestion, concentration and cell renewal, among other things.
Which hormones does the thyroid produce?
Thyroxine (T4), triiodothyronine (T3) and calcitonin. T4 and T3 control metabolism, calcitonin is involved in calcium balance.
What is the difference between T4 and T3?
T4 is the storage form with four iodine atoms and is hardly active itself. T3 has three iodine atoms and is the active hormone. Most T3 is only formed from T4 in the tissues.
Why is iodine so important for the thyroid?
Because iodine is built directly into the hormones. Without iodine, T4 and T3 cannot be formed. The body cannot make iodine itself.
What does the TSH level stand for?
TSH is the signal from the pituitary gland to the thyroid. A high value points to an underactive thyroid, a low value to an overactive one. It is the usual first lab value when a problem is suspected.
Can you recognise a thyroid disorder yourself?
No. The symptoms are non-specific and overlap with many other causes. Only a blood test brings clarity.