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A small gland in the neck
The thyroid is a butterfly-shaped gland, an organ that makes and releases chemicals. It wraps around the front of the windpipe, just below the voice box. Its job is to set how fast the whole body runs.
The Lab / Core medicine / Lesson 14
The thyroid axis.
A small gland in your neck sets how fast your whole body runs, and your brain keeps it in check with a brake. Build the loop one piece at a time, see the two ways it tips over, then watch four kinds of treatment each act on a different part.
Educational, not for clinical use
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Its hormone sets the body’s pace
The gland releases thyroid hormone into the blood. A hormone is a chemical messenger carried in the blood. This one reaches almost every cell and sets the metabolic rate: how fast cells burn fuel at rest, and so how much heat they make.
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Built from iodine
Thyroid hormone is built from iodine, a mineral we get from food such as iodized salt and seafood. Inside the gland, tiny hollow balls of cells called follicles pull iodine out of the blood and fix it onto a stored protein.
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Two forms: T4 and T3
The gland mostly releases T4 (thyroxine), which carries four iodine atoms, plus a little T3, with three. Cells around the body snip one iodine off T4 to make T3, the stronger form that does most of the work.
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The pituitary gives the order
The pituitary, a pea-sized gland at the base of the brain, sends TSH (thyroid-stimulating hormone) to the thyroid. TSH docks on the follicles and tells them to take up iodine and release T4 and T3. More TSH, more hormone.
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The hypothalamus starts the chain
Just above sits the hypothalamus, a small brain region that links the nerves to the hormones. It sends TRH (thyrotropin-releasing hormone) down to the pituitary, telling it to release TSH. The chain runs brain, pituitary, thyroid.
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Enough hormone puts on the brake
T4 and T3 in the blood report back to the brain. When they run high, the hypothalamus and pituitary release less TRH and TSH, so the thyroid slows. The result switches off its own cause: negative feedback.
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Too little hormone turns the order up
When T4 and T3 run low, the brake eases. The pituitary releases more TSH and pushes the gland harder until levels climb back. So TSH moves the opposite way to thyroid hormone, the key clue for reading the loop.
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It also speeds the heart
Thyroid hormone adds docking sites for catecholamines, the body’s adrenaline-like alarm signals (Lesson 6), so the heart and blood vessels answer them more strongly. More hormone: a faster, harder heartbeat. Less: a slower one.
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The whole loop: the key measures
Brain, pituitary and thyroid form one loop, held steady by the brake. It is read with thyroid function tests: blood tests of TSH and T4, often T3 too. Because of the brake, TSH usually moves opposite to T4 and T3.
What changes it
Hypothyroidism: an underactive gland
In hypothyroidism the gland makes too little hormone. The usual cause is Hashimoto thyroiditis: the immune system, the body’s defence against germs, attacks the gland. T4 and T3 fall, TSH climbs, and the body slows down and feels cold.
What changes it
Hyperthyroidism: an overactive gland
In hyperthyroidism the gland makes too much. In Graves’ disease, the usual cause, antibodies, immune proteins, latch onto the gland’s TSH docking sites and push it to grow and pour out hormone. The brake drives TSH down; the heart races.
What changes it
Levothyroxine: the missing hormone as a tablet
Levothyroxine is a lab-made hormone identical to the body’s own T4, taken as a tablet for hypothyroidism. It tops up what the damaged gland cannot make. Cells turn it into T3 as usual, and the brake brings TSH back down.
What changes it
Thionamides: the gland builds less
Thionamides, such as methimazole and propylthiouracil, are tablets that enter the gland and stop it fixing iodine onto its stored protein and joining the pieces into hormone. Less T4 and T3 is made, the brake eases and TSH rises back.
What changes it
Radioactive iodine: shrinking the gland
The thyroid pulls iodine from the blood, so radioactive iodine, a form that gives off damaging radiation, collects there and destroys overactive tissue. The gland shrinks, hormone falls and TSH rises back. Destroy too much and the gland becomes underactive.
What changes it
Beta-blockers: calming the heart only
Beta-blockers block the heart’s docking sites for alarm signals, so a racing heartbeat slows. They do not touch the gland: T4, T3 and TSH stay where the disease put them, so they ease symptoms while another treatment fixes the gland.
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Start with Next, or use the arrow keys. Pictures are illustrative, not measurements.
Reference values, beside the model.
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Normal teaching baseline.
The blue line on each bar shows where an illustrative teaching model sits, not a patient measurement, and it is never given a number. The shaded bands are the reference ranges quoted under each row. Selecting a disease or drug shows the expected direction of each value from the cited passage. For a drug, the direction compares it with the untreated disease it is used for, and the model applies the drug to that disease.
Sources
Run the loop yourself.
An illustrative teaching model of the settled loop. Pick a starting body, switch treatments on and off (they combine), and move the sliders. Dashed marks are the normal body; solid marks are your settings.
The brake curve: how TSH answers T4 (illustrative model)
How this teaching model works
Every level is worked out relative to the normal body and settles to a steady state; there is no time course. The pituitary’s TSH falls steeply as T4 in the blood rises (in the model, in proportion to T4 raised to the power −2.2), which is the negative-feedback brake; TRH falls with T4 too. The gland’s release is its working tissue × the share of its hormone-building it can still do × its iodine supply × the drive from TSH, which saturates at twice normal, plus any push from antibodies acting like TSH. T4 in the blood is the gland’s release plus any T4 from a tablet; T3 is made mostly from T4 in body cells and a little by the gland. The loop settles where these agree, found by halving the search range seventy times.
The body’s pace follows T3, and the heartbeat follows the pace; a beta-blocker blunts the heartbeat only. Gland size grows with working tissue, with antibody push and with a high TSH, which is how a goiter (a swollen gland) forms when iodine runs short. Each treatment moves the model in the direction of its cited mechanism: levothyroxine adds T4 from outside the gland, a thionamide blocks most of the gland’s hormone-building, radioactive iodine destroys about three quarters of its tissue, and a beta-blocker leaves the loop alone. The sizes of all these effects are illustrative choices; only their directions are meant to teach.
The model never turns its levels into lab numbers: the sources give only the direction of each change, so the readout says where each level sits against its healthy band, and the chart has no numbers at all; its shaded box is where TSH and T4 are both inside their healthy bands (the model’s normal sits in the middle of each band; for TSH, the middle on a ratio scale). The real reference numbers are in the Clinical values tab. Left out: binding proteins and free hormone, the weeks the loop takes to settle, pregnancy, illness, other medicines and every real-world difference between people. Nothing here is a prediction for a real person.