What is the thyroid?

Thyroid Health

Everything you need to know about your thyroid gland


Emily Condon
BSc (Hons) Biological Sciences

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What is the thyroid?

The thyroid is a butterfly-shaped endocrine gland which is found at the front of the neck, below the Adam’s apple. The gland consists of 2 lobes, each located either side of the windpipe and connected by a small thyroid tissue bridge called the isthmus. The main role of the thyroid is to produce key hormones that affect almost every cell in the body. Without these hormones, the body would be unable to convert nutrients into energy. The two main thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Dietary iodine is very important for thyroid health as it is used by the thyroid to produce T4 and T3. Iodine can be found in iodised salt, seafood, bread and milk. 

Why are thyroid hormones important?

The thyroid hormones T4 and T3 are vital in regulating metabolism. Whether it is determining how fast your heart beats or how quickly your intestines process your food, the thyroid hormones control the speed at which your body cells work. The thyroid produces and secretes a greater amount of T4 in comparison to T3. This is because the cells and tissues in the body convert T4 into T3 - the biologically active form of the hormone. The majority of T4 and T3 hormones that circulate in the blood are bound to proteins, while a smaller a proportion remains free. 

What controls the thyroid?

A feedback loop system involving the hypothalamus, pituitary gland and the thyroid controls the production and release of thyroid hormones. The pituitary gland is a small gland found behind the bridge of the nose while the hypothalamus is an area of the brain situated above the pituitary gland. If thyroid hormone levels are low, the hypothalamus produces and secretes TSH Releasing Hormone (TRH). TRH instructs the pituitary gland to produce the thyroid stimulating hormone (TSH), which in turn leads to an increase in T4 and T3 production. Levels of TSH continually rise and fall depending on the level of thyroid hormones in the blood. High TSH levels cause the thyroid to produce more thyroid hormone while low levels of TSH signal the thyroid to decrease hormone production. 

Learn more about thyroid disease:

What is thyroid disease?

What are the risk factors for thyroid disease?

What is hypothyroidism (an underactive thyroid)?

What is hyperthyroidism (an overactive thyroid)?

How is thyroid disease diagnosed?

Reverse T3 - what is it and do I need to test for it?

Reverse T3 - results explained 

What are the symptoms of thyroid disease?

Thyroid FAQs

Medicheck your thyroid

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