A gland shaped like a shield, from the Greek for the oblong shield, at the front of the neck
Thyroid gland — the name comes from thyreos, an oblong shield.
Anatomy · Glands & the immune system
Thyroid gland
The largest purely endocrine gland in the body
Endocrine system · front of the neck
A butterfly- or shield-shaped gland at the front of the neck, wrapped around the upper windpipe just below the larynx. Two lobes, one each side of the trachea, are joined across the midline by a narrow isthmus. It makes the iodine-containing hormones thyroxine (T4) and triiodothyronine (T3), which set the body's metabolic rate, and calcitonin, which lowers blood calcium. It is stuffed with hollow follicles filled with a pink colloid store.
How to spot them
- ▸Two lobes either side of the windpipe joined by a narrow bridge of tissue, like a butterfly or bow-tie
- ▸Sits below the thyroid cartilage (the Adam's apple) and moves up when you swallow
- ▸Under the microscope, ring-shaped follicles filled with pink colloid and lined by a single layer of cells
- ▸Very rich blood supply from a superior and an inferior thyroid artery on each side
- ▸The recurrent laryngeal nerve runs in the groove just behind it
What to know
Two lobes either side of the windpipe, joined across it by a narrow isthmus over the second to fourth tracheal rings
Thyroid gland — the isthmus lies in front of the upper trachea.
The only hormones in the body that need iodine to be made
Thyroid gland — thyroxine has four iodine atoms (T4), triiodothyronine three (T3).
Round sacs filled with pink colloid that stores thyroglobulin, seen in a stained section
Thyroid gland — the follicles are the histological signature.
After total removal, a patient must take levothyroxine tablets for the rest of their life
Thyroid gland — levothyroxine is synthetic T4. Surgery here also puts the recurrent laryngeal nerve at risk.
Lack of dietary iodine makes it swell into a visible lump at the front of the neck
Thyroid gland — the swelling is a goitre.
The surgeon Emil Theodor Kocher won the 1909 Nobel Prize in Physiology or Medicine for work on it (1909)
Thyroid gland — physiology, pathology and surgery.
Key ideas
- Follicular cells trap iodide from the blood, build it into the protein thyroglobulin and store it as colloid; when needed, the cells take the colloid back in and release T3 and T4.
- The pituitary's thyroid-stimulating hormone (TSH) drives the gland, and T3 and T4 feed back to switch TSH off — a classic negative-feedback loop.
Quick-fire facts
- •It is usually described as the largest purely endocrine gland in the body, weighing roughly 20–25 g in an adult
- •The two lobes are joined by the isthmus; a small pyramidal lobe rises from the isthmus in roughly half of people
- •Blood comes from the superior thyroid artery (from the external carotid) and the inferior thyroid artery (from the thyrocervical trunk of the subclavian)
- •The parafollicular or C cells, sitting between the follicles, make calcitonin
- •Because it is bound to the larynx and trachea by fascia, it rises and falls with swallowing — a useful way to tell a thyroid lump from other neck lumps
- •It develops from the back of the tongue: the thyroglossal duct marks its descent, starting at the foramen caecum
- •Its four small neighbours, the parathyroid glands, sit on its back surface and have a separate job
- •Emil Theodor Kocher (1841–1917), a Swiss surgeon, received the 1909 Nobel Prize in Physiology or Medicine for his work on the thyroid
- •Iodine deficiency is the world's commonest preventable cause of goitre; iodised salt was introduced to prevent it
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Sources
- OpenStax — The Thyroid Gland (Anatomy and Physiology 2e, section 17.4)
- NobelPrize.org — The Nobel Prize in Physiology or Medicine 1909
- NHS — Goitre