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Hormones are small molecules that participate in a wide range of body functions. Thyroid hormones are not the exception, since they regulate metabolism, among other mechanisms. When their balance is altered, the effects are certainly not good.

The thyroid gland: tiny but powerful 

The thyroid is a small two-inched gland that is located in the front of the neck, above the collarbone. Its main role is to produce hormones, which are molecules that act as messengers that take the message from where they are synthesized to other cells which can be right next to them or in a different organ.

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The thyroid secretes two iodine-containing hormones: thyroxin (T4) and triiodothyronine (T3). T4 is produced and released in higher quantities than T3 because the first one is converted into T3 when it reaches its target tissues.

Thyroid hormones: The energy drink of cells

Thyroid hormones have a very important role in our metabolism, the mechanism through which cells convert nutrients into energy or use energy to produce other molecules, such as proteins.

T3 and T4 stimulate cells to produce proteins and therefore, increase their need for oxygen.
Also, thyroid hormones participate in the metabolism of carbohydrates, fats, and proteins and in the stimulation of thermogenesis.

Thyroid hormones also play an important role in growth and development to the extent that they are essential for the normal development of the fetal brain. 

If this was not enough work load already, T3 and T4 affect other body systems as well. In the cardiovascular system, they increase heart rate and promote vasodilation; alterations in the balance of thyroid hormones in the central nervous system can cause mental problems and imbalances in the reproductive system can lead to infertility. 

How are thyroid hormones produced?

Thyroid hormones are produced in the thyroid follicles, where they are also stored until released. For the synthesis of these hormones, two components are required: tyrosine molecules, which derive from thyroglobulin, and iodine that we get from our diet.

Once both tyrosine and iodine are present in the gland, the enzyme thyroid peroxidase, is the one in charge of the production of the T3 and T4 hormones. What the peroxidase does is to attach the iodine molecule to the tyrosine and then convert them into the final products.  

Apart from iodine-containing hormones, the thyroid gland produces calcitonin as well, which controls the calcium levels in our body. 

The hypothalamus controls the production and release of thyroid hormones by the thyroid gland. 

In order to do this, the hypothalamus sends signals to the pituitary gland, which in turn controls the thyroid response through the thyroid stimulating hormone (TSH). 

But how does the pituitary and the hypothalamus know when it is necessary to start the production and release of thyroid hormones?

The level of thyroid hormones present in the circulation is the signal that promotes or inhibits the release of TSH.

Signals are sent to the hypothalamus regarding the state of T3 and T4 in the blood: if the levels are too low, it will send a message to the pituitary for it to release TSH, which will activate the production at the thyroid gland; if levels are too high, then the release of TSH will stop and therefore, the production of thyroid hormones will stop too. 

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