Anatomy Physiology of Thyroid GlandFitness Gear & Equipment
Anatomy Physiology of Thyroid Gland
By Mr Ghaz, January 7, 2011
Anatomy Physiology of Thyroid Gland
The thyroid gland is found in the neck, just below the level of the larynx. There are two lobes to the gland, and these lie just in front and at either side of the trachea as it passes down the front of the neck. The two lobes are connected by a small bridge of tissue, and there may be a smaller central lobe called the pyramidal lobe. In an adult, the gland will weigh about 20 gm (2/3 oz).
The function of the gland is to make the thyroid hormone, thyroxine. When the gland is looked at under a microscope, many small ‘follicles’ can be seen; these are islands of tissue containing collections of colloid, a protein substance to which thyroid hormone is bound and from which it can be released by enzymes.
It is not possible to tie the activity of thyroxine down to one specific thing. It is released from the gland and is then probably taken up from the blood into all the cells of the body. There appears to be a receptor on the surface of the cell nucleus that responds to the hormone. The overall effect of the hormone is to increase the amount of the energy that the cell uses; it also increases the amount of protein that the cell manufactures. Although the exact role of the hormone in the cell is not known, it is essential for life.
The thyroid gland contains iodine that is vital for its activity. This is the only part of the body that requires iodine and the thyroid is very efficient at trapping all the available iodine from the blood. An absence of iodine from the blood in the diet results in malfunction of the thyroid and the growth of the gland, a condition called endemic goiter.
Like, so many of the endocrine glands, the thyroid is under the control of the pituitary. When the pituitary produces TSH, it is increases the amount of thyroid hormone that is released from the gland. The amount of TSH that the pituitary produces increases if the amount of thyroxine circulating in the system falls, and decreases if it rises, which will result in a relatively constant level of thyroid hormone in the blood.
The pituitary is itself under the influence of the hypothalamus and the amount of TSH that is produced will be increased if there is a release of a substance called TRH (TSH releasing hormones) from the hypothalamus.
This situation is further complicated by the fact that thyroid hormone comes in two according to the number of iodine atoms that it contains.
Most of the hormone released from the gland is in the form of tetraiodothyronine, which contains four iodine atoms and is known as T4. However, the active hormone at the cell level is triiodothyronine, which contains three atoms and is known as T3. Although the gland releases some T3 into the blood, most of the output is T4, and this is converted into T3 in the tissues. Sometimes the tissues switch the way that they convertT4 to produce an ineffective compound called reverse T3. This means that there will be less thyroid hormone activity in the tissues even though the hormone level in the blood is adequate.
The parathyroids are four tiny glands situated behind the thyroid glands. They play a major part in controlling the levels of calcium in the body. Calcium is a vital mineral: not only because it is the major structural element in the formation of bones and teeth, but also because it plays a central role in the workings of the muscles and nerve cells. The body’s calcium levels have to be kept within fairly constant boundaries, otherwise the muscles stop working and fits may occur.
This is where the parathyroid glands come in: they keep the calcium levels in balance.
The absorption of calcium into the bloodstream is controlled by vitamin D, which we get from sunlight and some foods, and an important hormone produced by the parathyroids called parathyroid hormone or PTH. If the level of calcium is too low, the parathyroids secrete an increased quantity of the hormone, which actually releases calcium from the bones to raise the level in the bloodstream. Conversely, if there is too much calcium, the parathyroids reduce or halt the production of PTH, thus bringing the level down.
The parathyroids are so small that they can be difficult to find. The upper two are situated behind the thyroid gland, the lower two, however, may actually be inside the thyroid or occasionally right down inside the throat.