Article by Dr Manasa S, B.A.M.S
AIAPGET Exam Ready High-Yield Points
– The thyroid gland is a butterfly-shaped endocrine gland located anterior to the trachea at the level of C5–T1 vertebrae.
– A normal adult thyroid gland weighs 15–25 grams.
– The thyroid consists of two lateral lobes connected by an isthmus.
– The thyroid gland develops embryologically from the foramen cecum of the tongue.
– Thyroid follicles are lined by follicular cells, which synthesize T3 and T4.
– Parafollicular (C) cells of the thyroid secrete calcitonin.
– Thyroxine (T4) is the major hormone secreted, but T3 is the biologically active form.
– More than 99% of thyroid hormones are protein-bound in circulation.
– Only free T3 and free T4 are biologically active.
– Thyroid hormones increase basal metabolic rate (BMR) in almost all tissues except spleen and testes.
– Thyroid hormone receptors are nuclear receptors, making thyroid hormones genomic acting hormones.
– Thyroid hormone synthesis requires iodine, tyrosine, and thyroid peroxidase (TPO) enzymes.
– TSH from the pituitary is the most sensitive indicator of thyroid function.
– The thyroid gland is under control of the Hypothalamo–Pituitary–Thyroid (HPT) axis.
– Negative feedback by T3 and T4 suppresses both TSH and TRH secretion.
– Iodine deficiency is the most common cause of goiter worldwide.
– Graves’ disease is the most common cause of hyperthyroidism.
– Hashimoto’s thyroiditis is the most common cause of hypothyroidism.
– TSH-secreting pituitary adenoma causes secondary hyperthyroidism.
– Thyroid hormones are essential for brain development in fetal and early childhood life.
– Thyroid hormone excess increases heart rate, cardiac output, and oxygen consumption.
– Thyroid cancer most commonly arises from follicular epithelial cells, with papillary carcinoma being the most common type.
Thyroid Gland
The thyroid gland is an important endocrine gland that produces and releases hormones essential for regulating metabolism, growth, and energy utilization. Its primary role is to control the metabolic rate, which determines how the body converts food into energy. Since all body cells require energy to function, thyroid hormones influence almost every organ system.
When the thyroid does not function properly, it can affect the entire body. Thyroid disorders are common, usually treatable, and often lifelong, requiring careful monitoring.
Ayurveda Viewpoint to Understand Thyroid Gland, its functioning and related pathological manifestations
According to Ayurveda, the metabolic rate – which determines how the body converts food into energy – is determined by Agni (Pachaka Pitta). This Agni is active and balanced when it is controlled by balanced Samana Vayu. Thyroid gland is located in the region of Udana Vayu. Udana Vayu also has its influence on Nabhi i.e. Navel region which is the main seat of Pitta or Agni. Udana Vayu also controls the functions of Urja – energy and Bala – the strength required to run the body activities. Since Udana Vayu reaches Nabhi, which is the seat of Pachaka Pitta or Agni and since Samana Vayu is located very close to Agni, Udana Vayu and Samana Vayu are associated in the process of digestion, metabolism and metabolic rate. The Udana-Samana Axis determines the functional axis and territory of Thyroid hormones. Since the energy derived from food is distributed to all the cells and organs of the body, this part of function is carried out by Vyana Vayu. So, the Udana-Samana-Vyana Axis determines and monitors the functional axis of thyroid hormones.
When these Vata subtypes are relatively imbalanced and Agni too is disturbed, the resultant set of disorders reflect those caused by thyroid hormonal imbalances and dysfunctions.
Thyroid gland and its functions are controlled by Pituitary gland which in turn is controlled by Hypothalamus, both of which are located in the seat of Prana Vata. So, obviously the Prana Vata too comes into the axis.
Apana Vata too is put into the axis from a pathological viewpoint as thyroid dysfunctions cause diseases related to the reproductive system, which is governed by Apana Vata.
Anatomy of the Thyroid Gland
Location
The thyroid gland is located in the front of the neck, just below the larynx and in front of the trachea. It lies under the skin and muscles of the neck and straddles the windpipe.
The gland is butterfly-shaped, with two lateral lobes positioned on either side of the trachea. These lobes are connected by a narrow central bridge called the isthmus.
Size of the Thyroid Gland
The thyroid gland is approximately 2 inches long in adults. Under normal conditions, it does not protrude from the neck and is not visible.
Certain conditions can cause enlargement of the thyroid gland, a condition known as goiter.
Goiter is correlated with a condition called Galaganda in Ayurveda.
Parts of the Thyroid Gland
The thyroid gland consists of:
– Two lateral lobes
– A central isthmus connecting the lobes
Structurally, the gland is composed of two main cell types:
– Thyroid follicular cells (thyrocytes)
These cells synthesize and store thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3).
– Parafollicular cells (C-cells)
These cells secrete calcitonin, a hormone involved in calcium regulation.
Thyroid Follicles and Hormone Storage
The thyroid gland is unique among endocrine glands because it stores its hormones extracellularly. The follicular cells surround spherical structures called thyroid follicles, which contain colloids. Colloid serves as a storage reservoir for thyroid hormones, allowing the gland to maintain hormone supply even during periods of iodine deficiency.
What Does the Thyroid Gland Do?
The thyroid gland is an endocrine gland that produces and secretes hormones directly into the bloodstream. These hormones play a crucial role in regulating metabolism, growth, development, and overall physiological balance.
Cell Types of the Thyroid Gland
The thyroid gland is composed of two main types of cells, each with a specific hormonal function.
- Follicular cells (thyroid epithelial cells)
Follicular cells form the majority of thyroid tissue. These cells synthesize and secrete the thyroid hormones:
– Thyroxine (T4)
– Triiodothyronine (T3)
These hormones are primarily responsible for regulating metabolic activity in the body.
- Parafollicular cells (C cells)
Parafollicular cells produce the hormone calcitonin, which plays a role in regulating calcium and phosphate levels in the blood.
Hormones Secreted by the Thyroid Gland
As an endocrine gland, the thyroid produces and releases the following hormones:
– Thyroxine (T4)
T4 is the primary hormone produced by the thyroid gland. Although it is secreted in large quantities, it has relatively low metabolic activity. In peripheral tissues, T4 is converted into the more active hormone T3 through a process known as deiodination.
– Triiodothyronine (T3)
T3 is produced in smaller amounts by the thyroid gland but has a much greater biological effect on metabolism compared to T4. It is the most metabolically active thyroid hormone.
– Reverse triiodothyronine (rT3)
Reverse T3 is produced in very small quantities. It is an inactive form of T3 and counteracts the metabolic effects of active T3.
– Calcitonin
Calcitonin is secreted by parafollicular cells and helps regulate calcium levels in the blood by opposing the action of parathyroid hormone.
Role of Iodine in Thyroid Hormone Synthesis
Iodine is essential for the synthesis of thyroid hormones. It is obtained through diet, most commonly from iodized salt and drinking water.
The thyroid gland actively traps iodine from the bloodstream and incorporates it into thyroid hormones. Both iodine deficiency and iodine excess can disrupt normal thyroid hormone production and lead to thyroid dysfunction.
Physiological Effects of Thyroid Hormones
Thyroid hormones influence multiple systems in the body, including:
– Regulation of metabolism and energy utilization
– Control of heart rate
– Regulation of breathing
– Digestive activity
– Maintenance of body temperature
– Brain development and cognitive function
– Muscle function and strength
– Mental activity and alertness
– Skin and bone maintenance
– Regulation of menstrual cycles
– Maintenance of fertility
Because of these widespread effects, abnormalities in thyroid hormone levels can impact nearly every organ system.
What Other Organs and Glands Interact with the Thyroid?
The endocrine system functions as an integrated network of glands and hormones rather than isolated units. Many endocrine glands depend on signals from other glands to initiate or regulate hormone secretion. In addition, certain hormones can enhance or suppress the activity of other hormones to maintain balance.
The thyroid gland is closely regulated by higher endocrine centers and, in turn, influences multiple organ systems throughout the body.
Hypothalamic–Pituitary–Thyroid (HPT) Axis
The levels of thyroid hormones in the body are controlled through a tightly regulated feedback system involving the hypothalamus, pituitary gland, and thyroid gland.
– The hypothalamus, located at the base of the brain, secretes thyrotropin-releasing hormone (TRH).
– TRH stimulates the anterior pituitary gland to secrete thyroid-stimulating hormone (TSH).
– TSH acts on the thyroid follicular cells, stimulating the synthesis and release of thyroxine (T4) and triiodothyronine (T3), provided adequate iodine is available.
– Circulating T3 and T4 exert negative feedback on both the hypothalamus and pituitary gland, regulating further hormone release.
This axis ensures stable thyroid hormone levels necessary for normal physiological function.
Organ Systems Influenced by Thyroid Hormones
Thyroid hormones affect almost every organ system in the body.
Cardiovascular system
Thyroid hormones regulate cardiac output by influencing heart rate, strength of heart muscle contraction, and overall circulatory dynamics.
Nervous system
Thyroid dysfunction can lead to neurological symptoms. Hypothyroidism is commonly associated with depression, slowed cognition, and neuropathic symptoms such as numbness or tingling. Hyperthyroidism may cause anxiety, irritability, and restlessness.
Digestive system
Thyroid hormones influence gastrointestinal motility. Reduced hormone levels can cause constipation, while excess hormone levels may lead to increased bowel movements or diarrhea.
Reproductive system
Normal thyroid function is essential for reproductive health. Thyroid disorders can result in irregular menstrual cycles, ovulatory dysfunction, and reduced fertility in both men and women.
What Are the Early Warning Signs and Symptoms of Thyroid Problems?
Thyroid disorders can present with a wide range of symptoms, depending on whether the gland is overactive or underactive. However, because the thyroid plays a central role in regulating metabolism, heart rate, and body temperature, certain early warning signs are commonly seen across many thyroid conditions.
Some of the early symptoms that may suggest a thyroid problem include:
– A slow or rapid heart rate, which may be noticed as palpitations or fatigue.
– Unexplained weight gain or weight loss despite no significant change in diet or physical activity.
– Difficulty tolerating cold or heat, such as feeling unusually cold or excessively warm compared to others.
– Mood changes, including depression, low motivation, anxiety, or nervousness.
– Irregular menstrual periods, which may include heavy, scanty, or missed cycles.
These symptoms may develop gradually and are often mistaken for stress, aging, or lifestyle-related issues. If a person experiences one or more of these symptoms persistently, it is important to consult a healthcare provider. A simple blood test to assess thyroid hormone levels can help in early diagnosis and timely management.
Conditions and Disorders of the Thyroid
– Several conditions and disorders can affect the thyroid gland. Thyroid disease is very common. It is estimated that nearly 20 million people in the United States have some form of thyroid disorder. Women are affected more frequently than men and are about five to eight times more likely to be diagnosed with a thyroid condition.
– Thyroid diseases are broadly classified into primary and secondary disorders.
– In primary thyroid disease, the problem originates within the thyroid gland itself. For example, a thyroid nodule that produces excess thyroid hormones leads to primary hyperthyroidism.
– In secondary thyroid disease, the disorder originates outside the thyroid gland, most commonly in the pituitary gland. For instance, a pituitary tumor that secretes excess thyroid-stimulating hormone (TSH) can overstimulate the thyroid, resulting in secondary hyperthyroidism.
Major Thyroid Conditions
The four main conditions that affect the thyroid gland are:
– Hypothyroidism
– Hyperthyroidism
– Goiter
– Thyroid cancer
Hypothyroidism
– Hypothyroidism, also known as underactive thyroid, occurs when the thyroid gland does not produce enough thyroid hormones. As a result, metabolic processes in the body slow down.
– Hypothyroidism is a common condition and affects approximately 10 million people in the United States. It is generally treatable with appropriate medical care.
– Common causes of hypothyroidism include autoimmune disease such as Hashimoto’s thyroiditis, inflammation of the thyroid gland, iodine deficiency, congenital absence or dysfunction of the thyroid gland, excessive treatment for hyperthyroidism, and surgical removal of the thyroid gland.
Hyperthyroidism
– Hyperthyroidism, or overactive thyroid, occurs when the thyroid gland produces excessive amounts of thyroid hormones. This leads to an acceleration of metabolic processes.
– Hyperthyroidism affects approximately 1 in 100 people over the age of 12 in the United States and is also treatable.
– Common causes include Graves’ disease, thyroid nodules, thyroiditis, postpartum thyroiditis, excess iodine intake from diet or medications, excessive treatment for hypothyroidism, and benign tumors of the pituitary gland.
Goiter
– A goiter refers to an enlargement of the thyroid gland. Goiters are relatively common and affect about 5% of the population in the United States.
– The cause of a goiter depends on its type.
Simple goiters develop when the thyroid cannot produce enough hormones, leading the gland to enlarge in an attempt to compensate.
Endemic goiters occur due to iodine deficiency and are uncommon in countries where iodine is added to table salt.
Sporadic goiters have no identifiable cause in most cases, although certain medications such as lithium may contribute.
Thyroid Cancer
– Thyroid cancer is a malignancy that arises from thyroid tissue. Approximately 53,000 new cases of thyroid cancer are diagnosed each year in the United States. Most types of thyroid cancer have a good prognosis when detected early and treated appropriately.
– Thyroid cancer is classified based on the cell type involved.
Papillary thyroid cancer is the most common type, accounting for nearly 80% of cases.
Follicular thyroid cancer represents about 15% of cases.
Medullary thyroid cancer accounts for around 2% and is often associated with genetic mutations.
Anaplastic thyroid cancer is rare, aggressive, and comprises about 2% of cases.
Hypothyroidism vs Hyperthyroidism
| Feature |
Hypothyroidism |
Hyperthyroidism |
| Basic defect |
Deficiency of thyroid hormones |
Excess of thyroid hormones |
| Metabolic Rate |
Decreased basal metabolic rate |
Increased basal metabolic rate |
| Common cause (India / world) |
Iodine deficiency |
Graves’Disease |
| Common cause (Iodine sufficient areas) |
Hashimoto’s Thyroiditis |
Graves’ Disease |
| TSH level (primary disease) |
Increased |
Decreased |
| TSH level (secondary disease) |
Decreased / Normal |
Increased / Normal |
| Body weight |
Weight gain |
Weight loss despite increased appetite |
| Appetite |
Decreased |
Increased |
| Heat / cold tolerance |
Cold intolerance |
Heat intolerance |
| Skin |
Dry, coarse, cold |
Warm, moist, sweaty |
| Hair |
Dry, brittle, hair fall |
Fine hair, hair thinning |
| Heart rate |
Bradycardia |
Tachycardia |
| Blood Pressure |
Diastolic hypertension |
Systolic hypertension |
| Cardiac output |
Decreased |
Increased |
| Bowel movements |
Constipation |
Diarrhoea |
| Muscle tone |
Muscle weakness with slow reflexes |
Proximal muscle weakness |
| Reflexes |
Delayed relaxation |
Brisk reflexes |
| CNS effect |
Depression, lethargy |
Anxiety, irritability |
| Menstrual cycle |
Menorrhagia |
Oligomenorrhea / Amenorrhea |
| Fertility |
Decreased |
Decreased |
| Goiter |
May be present |
Common |
| Eye signs |
Usually absent |
Exophthalmos (Graves’) |
| Lipid profile |
Hypercholesterolemia |
Reduced cholesterol |
| Oxygen consumption |
Decreased |
Increased |
| Childhood presentation |
Cretinism |
Rare |
| Adult severe form |
Myxedema |
Thyroid storm |
| First-line screening test |
Serum TSH |
Serum TSH |
Risk Factors for Developing a Thyroid Condition
– Thyroid conditions are common and can occur at any age. However, certain factors increase the risk of developing a thyroid disorder.
– A family history of thyroid disease increases the likelihood of developing a thyroid condition.
– People with autoimmune disorders have a higher risk of thyroid disease. These conditions include type 1 diabetes, pernicious anemia, primary adrenal insufficiency, Sjögren’s syndrome, Turner syndrome, rheumatoid arthritis, and systemic lupus erythematosus.
– Use of medications that contain high amounts of iodine can affect thyroid function and increase risk.
– Increasing age, especially being over 60 years, raises the risk of thyroid disorders.
– Females are more commonly affected than males.
– A history of previous thyroid disease increases the risk of recurrence.
– Past thyroid surgery, such as thyroidectomy, or exposure to radiation therapy to the neck region also increases the risk of thyroid dysfunction.
Common Tests to Check Thyroid Health
– The most common first-line test to assess thyroid health is a blood test that measures thyroid-stimulating hormone (TSH).
– TSH testing is used as a screening tool for both hypothyroidism and hyperthyroidism.
– In general, the normal reference range for TSH is approximately 0.5 to 5.0 mIU/L. However, normal values may vary depending on the laboratory, age, and physiological conditions such as pregnancy.
– If TSH levels are abnormal, healthcare providers usually measure thyroid hormone levels, including thyroxine (T4) and triiodothyronine (T3), in the blood.
– These tests help determine whether the thyroid gland is underactive or overactive.
– When blood test results suggest structural abnormalities or nodules, imaging studies may be advised.
– Imaging tests include thyroid ultrasound, which evaluates the size and structure of the gland, and thyroid scans, which use small amounts of radioactive material to assess thyroid function and activity.
How Are Thyroid Conditions Treated?
Treatment for thyroid conditions depends on the type of disorder and its severity.
The three main treatment options include medication, surgery, and radiation therapy or chemotherapy.
Medication
– Medications are commonly used to manage both overactive and underactive thyroid conditions.
– Antithyroid medications reduce the ability of the thyroid gland to produce hormones and are primarily used to treat hyperthyroidism.
– Beta-blockers are used to control symptoms of hyperthyroidism, such as rapid heart rate and tremors, but they do not treat the underlying thyroid disorder.
– Radioactive iodine is used to damage thyroid cells, gradually reducing or eliminating thyroid hormone production. It is commonly used to treat hyperthyroidism and certain types of thyroid cancer.
– Thyroid hormone replacement medications are synthetic forms of thyroid hormones used to treat hypothyroidism.
– People who have had their thyroid gland surgically removed or destroyed by radioactive iodine usually need lifelong thyroid hormone replacement therapy.
Surgery
The most common surgical procedure for thyroid disorders is thyroidectomy, which involves removal of the entire thyroid gland.
– Thyroidectomy is a treatment option for several thyroid conditions and is the first-line treatment for most thyroid cancers.
– In some cases, a lobectomy may be performed, where only one lobe of the thyroid gland is removed.
Radiation Therapy and Chemotherapy
– Radiation therapy and chemotherapy are treatment options for certain types of thyroid cancer.
– These therapies work by destroying cancer cells and preventing their growth.
– Most thyroid cancers do not require radiation therapy or chemotherapy due to their favorable prognosis.
Care: How Can I Keep My Thyroid Healthy?
– Maintaining adequate iodine intake is essential for thyroid health, as iodine is required for thyroid hormone production.
– Most people receive sufficient iodine through iodized table salt and fortified foods.
– Dietary sources of iodine include dairy products such as cheese, cow’s milk, and yogurt, as well as eggs.
– Seafood sources of iodine include saltwater fish, shellfish, and seaweed.
– Plant-based sources include soy milk and soy sauce.
– Excess iodine intake should be avoided, as it can also lead to thyroid dysfunction.
– Individuals with concerns about thyroid health should consult a healthcare provider for guidance.
For further Reading:
Hypothyroidism Causes, Symptoms, Ayurvedic Treatment, Remedies
Thyroid gland – Latest research and news | Nature
Thyroid Gland – Recent articles and discoveries | Springer Nature Link
New insights in thyroid diagnosis and treatment – PMC
Emerging Therapies in Hypothyroidism – PMC
Advances in Thyroid Function Tests: Precision Diagnostics and Clinical Implications – PMC
Advancements in the Management of Endocrine System Disorders and Arrhythmias: A Comprehensive Narrative Review – PMC
