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Thyroid
Gray's Anatomy (1918) plate, public domain
The thyroid is a butterfly-shaped gland wrapped around the upper windpipe in the front of the neck. It makes the hormones thyroxine (T4) and triiodothyronine (T3), which set the body's metabolic pace — energy use, heat production, heart rate and growth. It needs iodine from the diet to build them. Its output is driven by pituitary TSH in a feedback loop, and it also contains C cells that make calcitonin.
Key facts
- The thyroid is one of the most richly perfused organs per gram in the body, which is why it can swell visibly when inflamed or overactive.
- It stores weeks' worth of hormone in its follicles — unusual among endocrine glands.
- T4 is largely a prohormone; tissues convert it to the more active T3 as needed.
- The gland moves upward when you swallow, a sign used in neck examination to confirm a lump is thyroid in origin.
What can go wrong
Thyroid disorders are common: underactivity (hypothyroidism, usually autoimmune Hashimoto's) causes fatigue, weight gain, cold intolerance and dry skin; overactivity (hyperthyroidism, often Graves' disease) causes weight loss, tremor, palpitations and heat intolerance. Enlargement is called a goitre. Lumps (nodules) are frequent and mostly benign, assessed with ultrasound and sometimes needle sampling; thyroid cancer is usually highly treatable.
Related conditions
Related procedures & tests
- Thyroid ultrasound
- Fine needle aspiration (FNA) cytology
- Thyroidectomy (thyroid removal)
- Hormone blood tests (thyroid, cortisol, reproductive)
Related symptoms
Related muscles
Watch: related videos
Frequently asked questions
What blood tests check the thyroid?
Usually TSH first, with free T4 (and sometimes T3 and thyroid antibodies) — the pattern distinguishes underactivity, overactivity and autoimmune causes.
What is a goitre?
Any enlargement of the thyroid gland, whether from iodine deficiency, autoimmune disease, nodules or inflammation; the gland's function may be high, low or normal.
Educational information only. This atlas explains normal anatomy and describes conditions in general terms; it is not medical advice and does not replace assessment by a qualified clinician. If you have symptoms that worry you, seek medical care.
