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The Thyroid Gland - NEET PG Physiology                                               

Feb 16, 2023

Thyroid Gland

The thyroid gland is an important topic in physiology because it plays a crucial role in regulating many physiological processes in the human body. The thyroid gland produces hormones that regulate metabolism and affect many systems in the body, including the cardiovascular system, the nervous system, and the digestive system. Additionally, thyroid dysfunction can result in various conditions such as hypothyroidism and hyperthyroidism, which can have significant impacts on a patient's health.

Furthermore, an understanding of thyroid physiology is essential for medical professionals as the thyroid gland is a common site of disease and is frequently involved in endocrine disorders. Knowledge of thyroid physiology and related disorders is therefore critical for the diagnosis, treatment, and management of patients.

Read this blog further to get a quick overview of this important topic for NEET PG/NExT exam preparations.


The thyroid gland is located in the neck and it secretes the Thyroid hormones. The central theme of Thyroid hormones is Basal Metabolic Rate as thyroid regulates the BMR.

Thyroid hormone is the only hormone synthesized in the extracellular site. Thyroid is not essential to life, but Parathyroid hormone is essential to life. There are two types of Thyroid hormones :- T3 & T4.

Synthesis of Thyroid hormone from Iodine

  • Iodine in the diet is absorbed by the blood stream. It is then trapped by the thyroid gland by the Iodide pump. This Iodide pump is also called NIS (Sodium Iodide Symporter) which carries 2 Na+ and 1 I-. The Iodide pump traps the iodine from the blood vessels which then enters the cell. 
  • On the other side of the cell, the ad-luminal or Apical membrane has another transporter called Pendrine, which is sodium independent. Now the Iodide enters the cell follicle and the iodine synthesis begins. Once the Iodide is transported to the follicle, it leads to oxidation of inorganic Iodide into free Iodine.
  • The enzyme acting as a catalyst for oxidation is Peroxidase. Iodine then combines with the tyrosine residues in the Thyroglobulin. If one iodine combines with tyrosine it is called Mono-Iodo-Tyrosine (MIT). When another iodine combines with MIT, it forms Di-Iodo-Tyrosine (DIT).
  • Now, one MIT & one DIT will couple up to form Tri-Iodo-Thyronine which is called T3 hormone. And, when one DIT will combine with another DIT it will form Tetra-Iodo- Thyronine which is called T4 hormone.

Release of Thyroid Hormones

  • Thyroid gland releases 90% of its hormones as T4, 9% as T3 and 1% reverse T3. Hence, T4 : T3 is 10 : 1, making T4 as its major secretion. T4 hormone enters every cell and under the influence of Di-iodinase enzyme within the cell, T4 is converted into T3. Action of the thyroid hormone is exerted by T3.

Why does thyroid gland majorly secretes T4?

  • Half – life of T4 is 7-8 days and of T3 is one day, Daily turnover rate of T4 is around 10%. However, it is 70% for T3
  • Plasma protein binding is more extensive for T4 as compared to T3.
    • Because of the above reasons, T4 forms a very stable pool of thyroid hormones in the body, making it the major secretion of the thyroid gland. 
  • In the blood, T3 & T4 are bound to 3 proteins namely TBG, TBPA and Albumin.

Metabolic & Non-Metabolic Actions of Thyroid Hormones 

T.H. performs metabolic & non-metabolic actions that are discussed below :- 

Metabolic Actions 

  • Thyroid controls the BMR, so, the thyroid hormone increases the metabolic rate of all the cells in the body. If the metabolic rate increases then the oxygen consumption will increase and heat production will also increase. BEA increase in Na+ - K+ ATPase (pump) activity is associated with increase in B.M.R. 
  • Thyroid hormones are known to cause lipolysis as it increases all aspects of carbohydrates, protein and fats. Thyroid hormones are known to cause relative vitamin deficiency because vitamins are cofactors for metabolism. 

Also Read :  Resting Membrane Potential - Physiology

Non-Metabolic /Systemic Actions 

  • As B.M.R increases under the influence of thyroid hormone, all aspects of the cardiovascular system (CVS) will get stimulated resulting in increased heart rate, increased blood pressure, and increased cardiac output. There is a potentiating effect of the thyroid hormone on catecholamines.  Because of this effect, thyrotoxicosis affected individuals have tremors as catecholamines have β receptors that increase and produce muscle tremors. 
  • Thyroid hormones are required for myelination in the nervous system in the first 6 years of life. Therefore, deficiency of these in 6 years of life results in improper myelination which causes Cretinism. The hallmark of cretinism is mental retardation.
  • Thyroid also increases synaptic excitability. Therefore, hyperthyroid individuals suffer from insomnia & hypothyroid individuals suffer from Somnolence (excessive sleeping). Thyroid hormones also cause increased motility in the GIT tract. 

Regulators of the Thyroid hormones  

  • Excess of TBH-TSH are important regulators from hypothalamus LTSH from the pituitary of the T.H. 
  • There are other regulators also that include: - 
    1. Temperature 
      • If you enter a cold environment, thyroid hormone secretion will increase which will also increase the B.M.R and heat, because of which you will face non-shivering thermogenesis in the cold. 
    2. Stress 
      • Stress also escalates the thyroid activity 
    3. High concentration of Iodides 
      • High amounts of iodides will block the further iodine uptake resulting in decreased thyroid hormone synthesis 
    4. Dietary goitrogens 
      • These are the foods that can produce goitre.
      • For e.g. cabbage is anti-thyroid 


Following are the disorders of thyroid hormones. 

  1. Hypothyroidism
    • It results in Cretinism in children leading to delayed milestones or mental retardation In adults, it results in Myxoedema as a result of GAG accumulation. Sub-dermal region & interstitium will absorb water on them making it jelly like consistent fluid causing Non-pitting type of Edema. It is seen on the shin (over tibia) called Pretibial Edema.
    • In addition to the above feature of Myxoedema, the hypothyroid individuals have a yellowish tint to their skin, because thyroid hormone is required to convert carotene into Vitamin A. which is not possible in case of hypothyroidism. Therefore, carotene will accumulate on the skin which will give it a yellow tinge. Additional features of Myxoedema may also include obesity, intolerance to cold, etc. 
    • Hyperthyroidism means excess secretion of thyroid hormones. When hyperthyroidism is accompanied by toxic symptoms, it is called thyrotoxicosis or Grave’s disease. The toxic symptoms or features mentioned include tremors, sleeping, heart rate > 90 beats/min. Grave’s disease is auto-immune in nature. 
    • Thyroid gland is stimulated by thyroid stimulating immunoglobulin (TSI) in the long run which results in hyperthyroidism. The hyperthyroid individual will show symptoms like a thin body, intolerance to heat, increased Heart rate and some special features like protrusion of the eyeball, called exophthalmos. 

Also Read: Symptoms of Hyperthyroidism

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