The thyroid gland (glandula thyreoidea) is a small, butterfly-shaped organ located at the base of our neck. It lies in the front, just under the voice box (larynx) and above our collar bone. It weighs about 20 to 60 grams on average in adults and is present in both women and men. The thyroid gland is part of our endocrine system. The hormone it produces is released into the blood stream and transported throughout the body, where they have an effect on various physiological processes.
Which hormones are produced by the thyroid gland?
The thyroid gland secretes three different hormones:
• Thyroid hormones – Triiodothyronine (T3) and Thyroxine (T4)
• Peptide hormone – Calcitonin
Hypothalamic-Pituitary-Thyroid Axis (HPT)
The hypothalamic–pituitary–thyroid axis (HPT) is a complex network between the thyroid gland, the pituitary gland (a small gland situated at the base of our brain), and the hypothalamus (a part of our brain). This neuroendocrine system is responsible for the release and regulation of the thyroid hormones (T3 and T4 ).
If the levels of thyroid hormones circulating in the blood is low, the hypothalamus senses it and responds to this by releasing thyrotropin-releasing hormone (TRH). The TRH in turn stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH). The TSH then signals the thyroid to produce thyroid hormone until the levels of T3 and T4 in the blood stream become normal. Hence, a coordinated balance between HPT forms a self-regulatory circuit and helps balance the level of thyroid hormones in our body.
Physiological Effects of the Hormones Produced by the Thyroid Gland
The thyroid hormones have an effect on various organ systems in our body. The central nervous system (CNS), the autonomic nervous system (ANS), the cardiovascular system, the respiratory system, the digestive system, and the musculoskeletal system, as well as the reproductive system, are all impacted by these hormones.
Thyroid hormones regulate crucial biological functions:
They play a major role in metabolism, which is a process by which body converts food into energy. Specific metabolic effects of thyroid hormones are on carbohydrates, lipid, and cholesterol synthesis. Thyroid hormones regulate the rate at which calories are burned, affecting weight loss or weight gain.
Thyroid hormones affects the heart rate, cardiac contractility, and cardiac output. Hence, can slow down or speed up the heartbeat.
Thyroid hormones can have an effect on mood. Hyperthyroidism causes anxiety and nervousness, whereas with hypothyroidism, the person suffers from mental sluggishness, and in severe cases, signs of depression.
They can increase or decrease sensitivity to environmental temperature. The thyroid produces hormones that are able to influence how much the blood vessels dilate. In turn, this affects how much heat can escape the body. An overactive thyroid may cause a person to feel too hot, while an underactive thyroid may cause a person to feel too cold. Basic metabolic rate also affects body temperature.
Thyroid hormones stimulate the respiratory centers and lead to increased oxygenation.
Thyroid hormones affect protein synthesis.
Thyroid hormones influences skeletal muscles contractility and myogenesis.
Thyroid hormones influence growth and development in children.
During the prenatal period, it is needed for the maturation of the brain.
Thyroid hormone affect fertility, ovulation, and menstruation. Hypothyroidism is associated with infertility.
Calcitonin helps to maintain blood’s calcium and phosphate levels
Euthyroid is the term used to describe a state of normal thyroid function in our body, so that the required amount of hormones is being produced and circulated in our bloodstream. Overactive or underactive thyroid leads to either over production or underproduction of the thyroid hormones.
Hyperthyroidism is a condition that occurs when our thyroid produces too much hormones. The common symptoms may include weight loss, accelerated heart rate, insomnia, anxiety and nervousness, palpitation, tremors, infrequent menstrual periods, and heat sensitivity. Too much thyroid hormones in the blood stream is termed thyrotoxicosis.
Hypothyroidism is the opposite, where the thyroid gland does not produce enough thyroid hormones. The symptoms could include weight gain, slower heart rate, fatigue, horse voice, heavy menstrual periods, depression, forgetfulness, dry skin, thinning hair, and intolerance to cold. Hypothyroidism may not cause noticeable symptoms in the early stages. Over time, untreated hypothyroidism can cause a number of health problems, such as obesity, joint and muscle pain, infertility, and heart disease, renal disease, myxedema (dermatologic changes that occur in hypothyroidism, and occasionally hyperthyroidism), and myxedema coma (extreme manifestation of hypothyroidism, potentially lethal).
There could be various reasons for hyper/hypothyroidism. Some common reasons are discussed below.
The presence of the thyroid and its various diseases have been noted and treated for a very long time; however, the gland itself has only been described and named since the 15th and 16th centuries.
Inflammation (swelling) of the thyroid is called thyroiditis. A person with thyroidiotis may present with symptoms of hyperthyroidism or hypothyroidism. There may be a phase of high thyroid hormone levels (thyrotoxicosis) followed by symptoms of an underactive thyroid gland, leading to hypothyroidism. Hashimoto’s thyroiditis is one such condition in which our own immune system attacks the thyroid glands and causes damage and inflammation. Thus, the damaged thyroid is unable to produce adequate amounts of thyroid hormones, leading to hypothyroidism. This disease may go undetected for a long time due to its slow progress. Though the disease itself is not a curable one, the symptoms of resultant hypothyroidism may be curbed by a thyroid supplement called levothyroxine.
Other types of transient thyroidiotis, which are temporary in nature and can be treated, are De Quervain’s (subacute) thyroiditis (commonly seen in women caused by viral infection like mumps or flu), postpartum thyroiditis (is rare and sometimes seen in women who have recently given birth), silent thyroiditis (affects both men and women), drug induced thyroiditis (some medicines used to treat certain cancers, heart conditions, and bipolar diseases may induce thyroiditis), radiation induced thyroiditis, and acute or infectious thyroiditis.
Graves’ disease is an autoimmune disorder that causes our thyroid to become hyperactive and produce more hormones than necessary. The body’s immune system releases abnormal antibodies that simulate the thyroid-stimulating hormone (TSH), which mistakenly signals the thyroid gland to overproduce. It is one of the most common thyroid problems, and the leading cause of hyperthyroidism. It is a treatable condition, but if left untreated, it can lead to serious complications.
– Thyroid cancer
Thyroid cancer can occur in any type of cells that the thyroid gland consists of, the most common being papillary cell carcinoma. Follicular and medullary cell carcinomas, thyroid lymphoma, and thyroid sarcoma are the other types of thyroid cancers. Most thyroid cancers have good prognosis and are generally considered curable, with the exception of anaplastic thyroid cancer (a rare form of aggressive thyroid cancer) that has poor prognosis.
Sometimes, nodules are found in the thyroid glands. Most of the time, the nodules do not cause any symptoms, secrete normal amount of thyroid hormones, and are non-cancerous. About 5% of these nodules may turn out to be cancerous. Sometimes, the nodules secrete excess thyroid hormone, which can be determined by blood test. Ultrasound and fine needle aspiration determine whether the cells are benign or cancerous.
Thyroid gland dysfunction at birth could cause various issues in children. Thyroid hormone deficiency present at birth, because of either iodine deficiency or a developmental defect in the thyroid gland (due to a genetic defect or of unknown cause), can lead to growth failure and permanent cognitive disability (cretinism), if untreated. A newborn with mild or moderate thyroid defiency may present with symptoms that may not be noticeable. Infants with severe thyroid deficiency have symptoms like jaundice, excessive sleepiness, less interest in nursing, poor muscle tone, hoarse cry, large protruding tongue, umbilical hernia, and low body temperature. If detected early during neonatal screening and treated correctly with thyroxine, most children with congenital hypothyroidism grow and develop normally, both physically and intellectually.
In pregnant women, during the first trimester, the baby depends on the mother for its supply of thyroid hormones, because he/she has yet to develop their own thyroid glands. The hormones are crucial for normal development of the baby’s brain and nervous system. During the prenatal period, a deficiency of thyroid hormones in the mother (either due to iodine deficiency or any other reason), if untreated, especially during the first trimester, may cause low IQ and problems with normal fetal development.
– Iodine Deficiency Thyroid Disorder
Iodine is an essential trace element our body requires. It is needed for the production of thyroid hormones. Its deficiency causes hypothyroidism and resulting symptoms. Endemic goiter is an iodine-deficiency disease (IDD) that occurs in those areas where the iodine content of the soil is so low that insufficient iodine is obtained through food and water, leading to conditions resulting from hypothyroidism. Numerous studies conducted in the past have shown that Nepal is a country having IDD in the endemic form distributed over all of our three geographical regions. In the population thus affected, goitre and intellectual and developmental disabilities were prevalent. In recent years, the cases have decreased due to the use of iodine-supplemented table salt. Other sources are seafood, certain vegetables, beans, and boiled eggs. However, studies have shown that excess iodine can also have adverse effects, depending on underlying thyroid function. Wolff–Chaikoff effect is the “presumed reduction in thyroid hormone levels caused by ingestion of a large amount of iodine”.
An enlarged thyroid gland is called a goitre. Goitre could be the result of various reasons, iodine deficiency leading to hypothyroidism being the most common. Autoimmune diseases involving the thyroid gland (Graves’ disease and Hashimoto’s thyroiditis), infection, inflammation, etc. could cause goitre. Sometimes, the whole thyroid gland becomes enlarged (diffuse goiter), and sometimes, individual lumps called nodules grow in the gland (nodular goiter).
Chemical Pollutants and Thyroid Function
Various studies have shown that various chemical agents have an influence on thyroid function. According to the US Environmental Protection Agency (EPA), an endocrine disrupting compound (EDC) is defined as “an exogenous agent that interferes with synthesis, secretion, transport, metabolism, binding action, or elimination of natural blood-borne hormones that are present in the body and are responsible for homeostasis, reproduction, and developmental process”. Some EDCs are associated with reduced thyroid hormone levels or impaired thyroid hormone action. A number of agents seem to increase the risk of autoimmune thyroid diseases, while others agents interfere with iodine uptake. Some may influence TRH and TSH secretion, and some act as thyroid hormone receptor agonists. Some examples are triclosan (found in antibacterial soaps and certain toothpaste), bisphenol A/BPA (found in plastic bottles/toys, etc.), isoflaves (found in soy products), cigarette smoke, phthalates (present in personal care products like shampoo, lotion, and even food products), and perchlorate (used in food packaging).
When to see a Doctor
Symptoms of the thyroid disorder develops over a period of time and may not always be noticeable at first. If we:
– notice any of the above-mentioned symptoms of hyper or hypothyroidism,
– if we have a family history of thyroid disease,
– if we have undergone radiation therapy for head and neck area,
– if we are taking any medicine that affects the thyroid functions,
– if we notice any lumps, and
– if we have had surgery to fully or partially remove the thyroid gland, then seeing a doctor is a good idea.
Modern science has acquired a better knowledge of thyroid glands, its biochemistry, its functions, its effects on our physiology, and its disorders. Many modern treatments and investigative modalities have evolved over time to better understand disorders of the thyroid and treat them.