What is the thyroid gland?
The thyroid gland is the largest endocrine gland in the adult body. It is usually located in the soft tissue below the front of the neck, with the center of the gland 12.5 cm from the superior sternal fossa, immediately in front of the thyroid cartilage and tracheal cartilage ring, on either side of the larynx. The thyroid tissue can grow ectopically, most commonly the posterior sternal thyroid gland, which can often compress the trachea when enlarged.
(A) Gross anatomy of the thyroid gland
The thyroid gland is divided into left and right lobes, connected by a narrower isthmus in the middle, which spans the upper part of the trachea in an “H” or butterfly shape. In adults, each lobe of the thyroid is about 3-5 cm long, 2 cm wide and 1-2 cm thick, while the isthmus is usually about 4 cm long, 5.5 cm wide and 2.0 mm thick; the adult thyroid weighs about 15-30 g. The superficial anterior aspect of the thyroid is composed of the cervical musculature, with the posterior medial aspect adjacent to the larynx and trachea, pharynx and esophagus, and the laryngeal nerve, and the posterior lateral aspect adjacent to the carotid sheath and the common carotid artery, internal jugular vein and vagus nerve within the sheath. The common carotid artery is the most medial, the vagus nerve is posterior, and the internal jugular vein is slightly anterior to the lateral side.
(B) Organization of the thyroid gland
The inner thyroid membrane covers the surface of the thyroid gland, and the connective tissue in the membrane penetrates into the parenchyma of the gland, dividing the parenchyma into many lobules of different sizes, each containing 20-40 follicles, which are the basic structures of the thyroid gland. The secretions of the follicular epithelium are colloids, mainly composed of thyroglobulin (TG), polysaccharides and enzymes. The follicles are interspersed with a small amount of connective tissue, abundant capillaries and groups of parafollicular cells, also known as C cells.
What are the physiological functions of the thyroid gland?
The physiological functions of the thyroid gland are mainly performed by its secretion of thyroxine (referred to as T3 and T4) and calcitonin.
(A) Physiological functions of thyroid hormones
The raw materials for synthesizing thyroid hormones are iodine and thyroglobulin. Organic iodine enters the thyroid follicular epithelial cells and is converted into active iodine by the action of peroxidase, and is rapidly synthesized into monoiodotyrosine (T1) and diiodotyrosine (T2) with tyrosine on thyroglobulin; two diiodotyrosines are coupled to form thyroxine (T4), and one diiodotyrosine and one monoiodotyrosine are coupled to form one triiodotyrosine (T3). The synthesis and secretion of thyroid hormones are regulated by the hypothalamus, pituitary gland and plasma levels of thyroid hormones to maintain a dynamic balance of plasma hormone levels, which is the hypothalamic-pituitary-thyroid axis system. Thyrotropin (TSH) is a glycoprotein secreted by the anterior pituitary gland, which is stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus and released. TSH is transported from the blood to the thyroid gland to promote the synthesis and secretion of T4 and T3; at the same time, an increase in serum T4 and T3 levels can inhibit the secretion of TSH, called negative feedback. Thyroid hormones have a wide range of physiological functions, mainly promoting material and energy metabolism and facilitating growth and development processes. (1) Thyroid hormones play a major role in the regulation of thermogenesis and energy metabolism in human body; (2) Thyroid hormones in physiological doses can regulate the metabolism of three major substances (protein, sugar and fat); (3) Thyroid hormones play an important role in promoting tissue differentiation, growth and maturation, and are essential hormones in maintaining normal growth and development, especially for the growth and development of the skeletal system and the nervous system.
(B) Physiological functions of calcitonin
C cells secrete calcitonin in a cytosolic manner. Its main physiological functions are to promote the activity of osteoblasts, to deposit calcium salts in bone, to deposit bone salts in bone-like material, and to inhibit the absorption of calcium in the renal tubules and the gastrointestinal tract, and to reduce the number and inhibit the activity of osteoclasts, thus lowering blood calcium.
What diseases can occur in the thyroid gland?
A variety of diseases can occur in the thyroid gland, from asymptomatic goiter to symptomatic changes in thyroid function, from non-neoplastic thyroid Hashimoto’s disease to neoplastic thyroid adenoma, and from nodular goiter, which has little effect on the body, to thyroid cancer, which can have life-threatening effects. Different diseases of the thyroid gland can have similar clinical manifestations, and the same clinical manifestations can be of completely different diseases.
Signs and symptoms of thyroid disorders
The most common sign of thyroid disease is goiter, which is a visible or palpable lump in the neck. The main symptoms of thyroid disease are symptoms related to thyroid hormone disorders.
①If thyroid hormone secretion is excessive, it increases the systemic metabolism and is called hyperthyroidism (hyperthyroidism), which manifests as weight loss despite increased appetite, increased heart rate, increased blood pressure, irritability, excessive sweating, protruding eyes, increased bowel movements, etc., sometimes accompanied by diarrhea, muscle weakness and hand tremors.
(ii) If there is insufficient secretion of thyroxine, it leads to hypothyroidism (hypothyroidism). Clinically, many cases of hypothyroidism are caused by surgical removal of the thyroid gland, long-term anti-thyroid medication or the use of radioactive 131 iodine for hyperthyroidism, which mainly manifests as facial swelling, dull gaze, fear of cold, dry skin with little sweating, roughness, drowsiness, poor memory, mental retardation and unresponsiveness.
(iii) Subacute thyroiditis occurs mostly after infection with influenza virus, mumps virus or measles virus, with mild to severe pain in the thyroid gland, which is tender to palpation and painful when swallowing and turning the head.
④ Acute purulent thyroiditis is less common and is mainly due to accumulation of purulent infection of the thyroid gland in the adjacent parts of the neck, manifesting as acute inflammatory features such as neck pain.
(⑤) Hashimoto’s disease can mostly manifest as hyperthyroidism in the early stages and hypothyroidism in the late stages.
(6) As for thyroid nodules (including thyroid cancer), they mostly have no obvious clinical manifestations unless the lesions are large and produce pressure on the surrounding tissues, which can lead to symptoms of compression of organs, esophagus, etc.
What are the causes of goiter?
Goiter is the most common sign of the most disorders of the thyroid gland and has a very diverse etiology. Depending on the etiology, the causes of goiter are.
① Hashimoto’s disease and toxic goiter, which are the two most common clinical causes of goiter.
② iodine deficiency and high iodine: iodine deficiency is the main cause of endemic goiter, mostly found in the interior and mountainous areas away from the high terrain of the ocean. Currently, iodine supplementation with edible iodized salt is commonly used in China, which effectively controls the incidence of goiter; due to long-term excessive iodine intake, excessive inorganic iodine ions in the thyroid tissue cause goiter through increased secretion of thyroid-promoting hormones.
(iii) Thyroid nodules and thyroid cancer, if a single lesion if large in size, can lead to an asymmetrical thyroid, and if there are more lesions can also cause a symmetrical enlargement of the thyroid.
Overview of thyroid cancer.
Thyroid cancer is a malignant tumor of the thyroid gland. The more definite cause is a history of radiation exposure, other possible causes are dietary factors (high iodine or iodine deficiency diet), increased estrogen secretion and genetic factors; in addition, the incidence of thyroid cancer in patients with Hashimoto’s disease is 6 times higher than that of other patients. Thyroid cancer is generally divided into differentiated thyroid cancer including papillary thyroid cancer and follicular thyroid cancer, low differentiated thyroid cancer such as medullary carcinoma and undifferentiated thyroid cancer, and some rare malignant tumors such as thyroid lymphoma, metastatic thyroid cancer and squamous thyroid cancer. Among them, the proportion of papillary thyroid cancer is about 90%, follicular thyroid cancer is about 5%, medullary thyroid cancer is about 4%, and the rest are other malignant tumors such as undifferentiated thyroid cancer.
What tests are available for thyroid disease?
When a thickening or lump is found in the neck, if the enlargement is symmetrical it is usually caused by a diffuse thyroid lesion, if the neck is raised or enlarged asymmetrically, it is usually a thyroid nodule. If there is an enlarged thyroid gland or a thyroid mass, further tests are usually needed to determine the nature of the thyroid disease, such as blood tests to check thyroid function and, if necessary, radionuclide and ultrasound examinations of the thyroid gland, or even cytology by thyroid aspiration.
(i) Laboratory tests
When hyperthyroidism occurs, the TSH level in the blood circulation is low, while the T3 and T4 levels are elevated; in hypothyroidism, the T3 and T4 levels are reduced, while the amount of TSH in the blood circulation is increased; in Hashimoto’s disease, although the T3 and T4 levels may be normal, elevated or decreased, the anti-peroxidase antibodies and/or anti-microsomal antibodies are increased. It is important to note that there is a lack of laboratory markers for thyroid cancer.
(ii) Nuclear medicine examination
Radioactive iodine or technetium isotope scan (ECT) is an important tool to determine the functional size of thyroid nodules. According to the American Thyroid Association, ECT findings include hyperfunctional (higher uptake than surrounding normal thyroid tissue), isofunctional or warm nodules (same uptake as surrounding tissue), or nonfunctional nodules (lower uptake than surrounding thyroid tissue). High-functioning nodules have a low rate of malignancy, and nodules need to be evaluated if the patient has significant or subclinical hyperthyroidism. Nodules should be evaluated if serum TSH levels are high, even if they are only at the highest limit of the reference value, as this is when nodules have a higher rate of malignancy. However, ECT often does not reveal nodules smaller than 25px or microscopic cancers, so ECT should not be used for such nodules.
(iii) Ultrasound examination
Due to the development of ultrasound medicine, ultrasound technology has been able to make accurate evaluation and diagnosis of most thyroid diseases. For example, toxic goiter is characterized by symmetrical enlargement of the thyroid gland with hypoechoic parenchyma and abundant blood flow signal; Hashimoto’s disease is characterized by symmetrical enlargement of the thyroid gland with many patchy hypoechoic or hyperechoic cords in the parenchyma and normal blood flow signal; nodular goiter is characterized by symmetrical or asymmetrical enlargement of the thyroid gland, and the thyroid gland may not be enlarged, but many hypoechoic nodules of different sizes are seen in the parenchyma. Thyroid adenoma is a well-defined isoechoic or hyperechoic nodule with surrounding blood flow signal or abundant internal blood flow signal; thyroid cancer is usually a hypoechoic nodule, but the border is usually blurred, the shape may be irregular, and it may be accompanied by small calcified spots. It should be noted that ultrasound can diagnose about 80% of thyroid cancer, and can make accurate judgment on lesions below 5mm, and can also evaluate whether there is metastasis in the lymph nodes of the neck, which is very helpful for clinical diagnosis and treatment plan of thyroid cancer.
(D) Puncture (FNAC) examination
For some thyroid nodules that are difficult to determine benign or malignant by ultrasound, fine puncture cytology (FNAC) can be performed. This technique, if performed under ultrasound guidance, will greatly increase the success rate of puncture and improve the diagnosis.
How are thyroid disorders managed?
(A) Clinical management of thyroid disorders
For thyroid disorders with abnormal thyroid function, conventional treatment conventional treatment often provides effective treatment by reducing elimination or supplementation with hormonal agents. For example, treatment of hyperthyroidism requires suppression of hormone production, while hypothyroidism requires hormone supplementation. The choice of which therapy to choose will be evaluated by the physician based on thyroid function, age, general condition and medication history. Although subacute thyroiditis can cause temporary hyperthyroidism, this does not require treatment and any pain associated with inflammation of the thyroid can be relieved with acetaminophen or aspirin. Surgery may be considered in cases where medication has failed, but the patient should be younger than 45 years of age. Occasionally, hypothyroidism can occur in patients with medically treated hyperthyroidism. In hypothyroid and thyroidectomized patients, neither surgery nor conventional medications can increase thyroxine production and hormone replacement therapy is required for life. Surgical resection of thyroid cancer is often performed, and if the cancer has spread to any tissue other than the thyroid gland, such as the lymph nodes in the neck, it can be removed as well. For benign thyroid nodules, surgery should be considered only if the mass is large enough to cause pressure on the surrounding organs; for smaller nodules, follow-up ultrasound examination can be considered to observe whether there is any change in the nodule; of course, if the patient is psychologically burdened, surgery can also be considered for smaller nodules.
(B) Dietary contraindications for thyroid disease
Many patients may ask what to eat or what not to eat after suffering from thyroid disease. My answer is: ①Patients with hyperthyroidism should avoid spicy foods, tobacco and alcohol, leeks, raw onions, ginger, garlic and fried foods, which may promote the development of hyperthyroidism; ②Patients with hyperthyroidism should be careful with foods containing too much iodine, such as kelp, seaweed and other products, which will affect the doctor’s judgment and analysis of the disease and will interfere with clinical treatment; ③Patients with hypothyroidism should not eat too much cold, such as ice cream, popsicles, ice water, chilled food, etc.; ④Patients with hypothyroidism should not eat too much cold food. ③Patients with hypothyroidism should not consume cold food, such as ice cream, popsicles, ice water and chilled food, etc.
How do you think about thyroid disease?
(a) Don’t worry too much about benign thyroid diseases
Hyperthyroidism, hypothyroidism, Hashimoto’s disease, subacute thyroiditis, nodular goiter and thyroid adenoma are all benign diseases that do not cause fatal loss of life and will not have much impact on life as long as the necessary treatment and follow-up are carried out in cooperation with the doctor. It is important to point out that for benign thyroid nodules only regular follow-up is needed, and malignant changes usually do not occur; many patients choose unnecessary surgical procedures, resulting in the need for lifelong thyroid hormone replacement therapy after surgery.
(B) Don’t be afraid of thyroid cancer
In recent years, the incidence of thyroid nodules and thyroid cancer has been increasing year by year, which has caused many patients to panic. In fact, thyroid cancer belongs to malignant tumors that are less harmful to human body and usually do not affect life unless they are in advanced stage. Although thyroid nodules are very common in the population, thyroid cancer patients only account for about 10% of thyroid nodule patients; and most of the thyroid cancers found clinically are microscopic cancers (<1.0cm), which belong to the early stage of cancer, so if surgical treatment has been performed at this time, what else should we worry about? The American academic system even believes that thyroid cancer less than 5 mm can be treated as benign tumor and only requires clinical follow-up without surgery. It should be noted that thyroid cancer usually does not recur after surgery and does not have a major impact on the body and only requires lifelong thyroxine replacement therapy.
Even thyroid cancer is just a paper tiger, as long as you treat it correctly and follow up with the necessary treatment as recommended by your doctor.