The Thyroid Gland is a butterfly-shaped gland found in the lower front part of the neck. Its lobes (the butterfly wings) wrap around the trachea and are connected at the centre by an Isthmus. The thyroid gland’s primary function is to secrete hormones that regulate our metabolic rate, protein synthesis and growth. These notes will outline the Physiology of the Thyroid Gland, and its hormones for nursing and health care students.
The Thyroid Gland produces and stores two hormones:
Thyroid Hormone: made up of Thyroxine (T4) and Triiodothyronine (T3)
Physiology of The Thyroid Gland Part 1: Thyroid Hormones
Our cells are constantly breaking down, reproducing and performing essential functions to keep us alive. This process of constant work is called cellular metabolic activity, and it is the job of the Thyroid Hormones to regulate it. Essentially what the Thyroid Hormones do, is that they increase the speed of these metabolic activities, and they do this in two ways.
Firstly, they increase the number of enzymes that supply oxygen to the cells, allowing the cells to work more easily. Imagine yourself going to the beach and deciding to go snorkelling. If you go out using only the mask you will need to go up for air every couple of seconds. But, if you have an air pipe that allows you to breathe in the air whenever you want, then you can be snorkelling without any interruptions for as long as you want.
Secondly, the thyroid hormones alter the sensitivity of the tissues to other hormones. Again making them work more quickly and easily.
By doing these two mechanisms, the Thyroid Hormones affect nearly every major organ in our body because they affect :
Basal metabolic rate
Serum cholesterol levels
Physiology of The Thyroid Gland Part 2: Production of Thyroid Hormones
The two hormones making up the thyroid hormone (T3 & T4) are both amino acids, and both of them have iodine atoms embedded in their structure. T3 has three iodine atoms, while T4 has four iodine atoms.
Their production is relatively simple, and it all starts with digestion. As we eat and food goes into our GI tract, our bodies absorb the Iodide out of the GI tract and into the circulatory system. Once the blood reaches the thyroid gland, Iodide is taken up and concentrated within the thyroid cells. At this point, the thyroid gland converts the Iodide into Iodine, which then reacts with the amino acid Tyrosine to produce T3 and T4.
Physiology of The Thyroid Gland Part 3: Regulation of Thyroid Hormones
Once T3 and T4 are produced, they are stored within the Thyroid Gland. Their secretion out of the gland is controlled by Thyrotropin Stimulating Hormone (TSH) a hormone produced in the Anterior Pituitary Gland.
The control and release of thyroid hormone is regulated by the TSH through a negative feedback mechanism, which means that there is a loop. When the body notices a low concentration of Thyroid Hormone in the blood, it signals the Anterior Pituitary Gland to release TSH. This increases the concentration of Thyroid Hormone in the blood, and once it reaches a normal level of concentration known as Euthyroid, the body signals the Anterior Pituitary Gland to stop releasing TSH which in turn starts lowering the concentration of Thyroid Hormone in the blood. And the process repeats itself, over and over again to maintain balance.
The binding and transportation of T3 and T4 is carried out by three thyroid-binding hormones:
Thyroxine Binding Globulin (TBG)
Physiology of The Thyroid Gland Part 4: Calcitonin
The other type of hormone that is produced and stored in the thyroid gland is Calcitonin and while it’s often overlooked, Calcitonin has an important function. It is responsible for reducing the level of calcium in the blood and depositing it in our bones.