Thyroid disorders have now become a very common disease. For an average patient, most of them learn about it from a physical examination in the hospital, or they unknowingly find a lump in their neck and come to the clinic for further treatment. The main purpose of this article is to give you a general understanding of the thyroid gland and its diseases, so that if you learn that you have a thyroid disease, you will not panic and will actively cooperate with our doctors for further treatment.
The structure of the thyroid gland
The thyroid gland is the largest endocrine gland in the body. It is brownish-red in color, divided into two lobes and connected in the middle (called the isthmus), and is “H” shaped, with about 20 to 30 grams. The thyroid gland is located on the anterior side of the upper part of the trachea in the lower part of the larynx and can move up and down with the larynx when swallowing. Although the iodine content of the gland is usually 25 to 50 times higher than that of the blood, 1/3 of the daily dietary iodine intake enters the thyroid gland, and 90% of the total body iodine content is concentrated in the thyroid gland. Thyroid hormone is a hormone secreted by the thyroid gland.
The thyroid gland is an important organ of the endocrine system, which is distinctly different from other systems of the human body (such as the respiratory system), but is closely linked with the nervous system, interacting and cooperating with each other, and is known as one of the two major biological information systems, without which the internal environment of the body cannot maintain relative stability.
Most people do not know where the thyroid gland is located, but most people are not unfamiliar with “thick neck disease”, which is actually an enlarged thyroid gland, which tells us that the thyroid gland is located in the neck. To be more specific, the thyroid gland is located about 2 to 3 cm below the “laryngeal node”, which we can touch ourselves, and can move up and down with it when swallowing something.
The thyroid gland is shaped like an “H”, brownish red, with two lateral lobes, left and right, connected by an isthmus. The two lateral lobes are attached to the lower part of the larynx and the upper side of the trachea, reaching the middle of the thyroid cartilage and the sixth tracheal cartilage, with the isthmus mostly located in front of the second to fourth tracheal cartilage, and some people are not developed. Sometimes a cone-like lobe protrudes upward from the isthmus, varying in length, with the longest reaching the hyoid bone, a vestige of embryonic development that often degenerates with age, making it more common in children than in adults.
The thyroid gland is covered with a fibrous capsule, called the thyroid capsule, which extends into the glandular tissue and divides the gland into lobes of varying sizes.
The thyroid gland matures during puberty and weighs 15 to 30 grams. The width of each of the two lateral lobes is about 2 cm and the height is 4 to 5 cm, and the width of the isthmus is 2 cm and the height is 2 cm. The thyroid gland is slightly larger in women than in men. Under normal circumstances, the thyroid gland is neither visible nor palpable in the neck because it is very small and thin. If the thyroid can be felt in the neck, even if it is not visible, the thyroid is considered to be enlarged. This degree of enlargement is often physiological, especially in women during puberty, and is not usually the result of disease, but can sometimes be pathological.
The thyroid gland is composed of many follicles. What is seen microscopically: the follicles consist of simple cuboidal glandular epithelial cells surrounded by a central follicular cavity. The adenosepithelial cells are the site of thyroid hormone synthesis and release, and the follicular cavity is filled with a homogeneous gelatinous substance that serves as a thyroid hormone complex and a reservoir for thyroid hormones. Changes in follicular morphology reflect the functional state of the gland: when glandular activity is reduced, the glandular epithelium is flattened and the storage in the follicular lumen is increased; if activity is hyperactive, the follicular epithelium is columnar and the storage in the follicular lumen is reduced.
Thyroid hormones and their biological effects
Two biologically active hormones secreted by the thyroid gland are thyroxine (also known as tetraiodothyronine (T4)) and triiodothyronine (T3). They are a group of iodine-containing tyrosines, which are synthesized in the thyroid gland cells using iodine and tyrosine as raw materials. The thyroid gland cells have a strong capacity to take up iodine. The human body takes in 100 to 200 μg of iodine from the diet every day, of which about 1/3 enters the thyroid gland. The total iodine content of the thyroid gland is about 8,000 μg, accounting for 90% of the total body iodine content, indicating that the thyroid gland has a strong iodine pumping capacity. When the thyroid gland is hyperactive, the iodine pumping capacity exceeds normal and iodine intake increases; when it is low, it is lower than normal and iodine intake decreases. Therefore, the ability of the thyroid gland to take up radioactive iodine (131I) is used clinically as one of the methods to routinely check thyroid function.
After iodine ions are taken into the epithelial cells of the thyroid follicles, they are rapidly oxidized to activated iodine by the action of peroxidase, and then the tyrosine residues in thyroglobulin are iodinated by the action of iodase to produce monoiodotyrosine (MIT) and diiodotyrosine (DIT). In this way, thyroglobulin containing the four tyrosine residues is stored in the follicular compartment.
When the thyroid gland is affected by TSH and thyroid hormone is released, the glandular epithelial cells first swallow the thyroglobulin in the follicular cavity into the glandular cells by swallowing, and under the action of lysosomal protein hydrolase, the thyroglobulin is broken down. The number of T4 on the thyroglobulin molecule is much higher than that of T3, so T4 accounts for about 90% of the total amount of hormone secreted, while T3 is secreted in a smaller amount, but its activity is large, five times that of T4. The total amount of T4 secreted daily is about 96 μg, while that of T3 is about 30 μg. After T4 is released into the blood, part of it is combined with plasma proteins, while the other part is transported in the blood in a free state, and the two can be transformed into each other to maintain T4 and T3 are in dynamic balance in the blood, because only the free form can enter the cells to play a role. t3 is released into the blood, because the affinity with plasma proteins is small, mainly in the free form. About 50% of T4 is deiodinated into T3 every day, so the role of T3 cannot be ignored.
The biological roles of thyroid hormones are mainly in the following three areas.
(I) Promoting growth and development
The most obvious role of thyroid hormone in promoting growth and development is during infancy, with the greatest impact during the first four months of life. It mainly promotes the growth and development of bones, brain and reproductive organs. Without thyroid hormone, the pituitary gland’s GH cannot function either. Moreover, in the absence of thyroid hormone, the pituitary gland also produces and secretes less GH. Therefore, congenital or early childhood deficiency of thyroid hormone causes cretinism. Patients with cretinism have short stature due to stagnant bone growth, and the ratio of upper and lower body lengths is abnormal, with the upper body taking up a larger proportion than normal. In addition, the growth of dendrites, axons, myelin sheaths and glial cells of nerve cells is impaired, and the brain is underdeveloped, resulting in mental retardation. They also fail to develop mature sexual organs. Patients must be supplemented with thyroid hormones around three months after birth; later than this period, treatment is often ineffective.
(II) Effects on metabolism
1. The thermogenic effect of thyroid hormone can increase the oxygen consumption rate of most tissues and increase the thermogenic effect. This thermogenic effect may be due to the fact that thyroid hormone can increase the synthesis of Na+-K+ pump on the cell membrane and can increase its vitality, the latter being an energy-consuming process. Thyroxine increases the basal metabolic rate, and 1mg of thyroxine can increase heat production by 4000 KJ. The basal metabolic rate of patients with hyperthyroidism can be increased by about 35%; while the basal metabolic rate of patients with hypothyroidism can be reduced by about 15%.
2, the effect on the metabolism of the three major nutrients it is very complex impact on the metabolism of the three major nutrients. In general, under normal conditions, thyroid hormone mainly promotes protein synthesis, especially protein synthesis of bone, skeletal muscle and liver, which is important for growth and development in early childhood. However, excessive secretion of thyroid hormone causes protein breakdown, especially in skeletal muscle, resulting in wasting and weakness. In terms of glucose metabolism, thyroid hormone has the effect of promoting the absorption of sugar and the breakdown of liver glycogen. It also promotes the utilization of sugar by peripheral tissues. In short, it accelerates sugar and fat metabolism, especially promoting the decomposition and oxidation process of sugar, fat and protein in many tissues, thus increasing the oxygen consumption and heat production of the body.
(III) Others
In addition, thyroid hormone has an important role in the activity of some organs. It is important for maintaining the excitability of the nervous system. Thyroid hormone can act directly on the heart muscle to promote the release of Ca2+ from the sarcoplasmic reticulum, resulting in increased contraction of the heart muscle and faster heart rate.
Examination methods of thyroid gland
(a) Visual examination The size and symmetry of the thyroid gland are observed. The thyroid gland does not protrude in normal people, but may increase slightly in women during puberty. If it is not easy to identify the thyroid gland, ask the patient to put both hands behind the pillow and tilt the head back, and then observe it.
(B) Palpation
1. Thyroid isthmus: The thyroid isthmus is located in front of the second to fourth tracheal ring below the cricoid cartilage. The soft tissue in front of the trachea can be felt by touching it with the thumb in front of the subject or with the finger behind the subject from the upper sternal notch upwards to determine whether there is thickening or not.
2, lateral thyroid lobe: front palpation: one hand thumb pressure on one side of the thyroid cartilage, push the trachea to the opposite side, the other hand show finger, middle finger in the opposite side of the posterior edge of the sternocleidomastoid muscle forward pushing the lateral thyroid lobe, thumb in the anterior edge of the sternocleidomastoid muscle palpation, with swallowing action, repeat the examination, you can touch the pushed thyroid gland. Use the same method to examine the other side of the thyroid. Posterior palpation: similar to the anterior palpation. The thumb of one hand applies pressure to the thyroid cartilage on one side, pushing the trachea to the opposite side. The thumb of the other hand pushes the thyroid gland forward at the posterior edge of the sternocleidomastoid muscle on the opposite side, and the thyroid gland is palpated by the thumb and middle finger at its anterior edge. Repeat the examination with the swallowing motion. Examine the thyroid gland on the other side in the same way.
(c) Auscultation When an enlarged thyroid is palpated, a bell-shaped stethoscope placed directly on the enlarged thyroid gland can be helpful in diagnosing hyperthyroidism if a low-pitched continuous venous “buzzing” sound is heard. In addition, a systolic arterial murmur may be heard in diffuse goiter with hyperfunction.
(d) The enlarged thyroid gland can be divided into three degrees: degree I if the enlargement is not visible but palpable; degree II if the enlargement is visible and palpable but within the sternocleidomastoid muscle; and degree III if it exceeds the outer edge of the sternocleidomastoid muscle.