The thyroid is a butterfly-shaped gland that sits in the neck, just below the Adam’s apple. The thyroid gland is made up of two lobes (the wings of the butterfly), connected by a bridge that sits in front of the trachea (or wind pipe). The thyroid gland produces thyroid hormone, which is released into the bloodstream to help the body regulate its metabolic rate.
In simple terms, the thyroid gland responds to signals from the brain to produce thyroid hormone, to instruct the body’s metabolism to speed up or slow down.
In the brain, the hypothalamus incorporates signals from the body. When the rate of metabolism (breakdown of fats, proteins and carbohydrates from the diet and body stores) is too low, it releases thyrotropin releasing hormone (TRH) into the circulation, which acts locally on the pituitary gland. The pituitary then produces Thyroid Stimulating Hormone (TSH), which then enters the circulation to reach the thyroid gland in the neck. In response to TSH, the thyroid gland produces thyroid hormone, which can then be shared with the rest of the body to increase metabolic rate. If the metabolism is moving too quickly, thyroid hormone signals back to the hypothalamus, to produce less TRH and slow down the process again.
If this system (called the Hypothalamic-Pituitary-Thyroid Axis) is not working correctly, there may be an inappropriate amount of thyroid hormone produced, leading to hypothyroidism – where there is too little thyroid hormone; or hyperthyroidism - where there is too much thyroid hormone.
As thyroid hormone is responsible for managing the rate of metabolism, all systems in the body are affected by its actions. In particular, thyroid hormone has important effects on:
Brain function and learning
Digestion and bowel movements
Heart rate and cardiovascular health