Thyroid gland and its functions: Revision notes for NEET 2023 Biology

The thyroid gland is a small butterfly-shaped endocrine gland present in our neck. Its primary function is the production and secretion of thyroid hormones, which are indispensable for the body’s normal functioning. Their deficiency or excess leads to the development of a variety of medical conditions, affecting almost all the organs of the body. The thyroid cells are unique as they can take up iodine from the blood and then convert it into thyroid hormone in several steps. Also, it is interesting to know that the thyroid gland is the body’s largest endocrine gland.

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Location of the thyroid gland

To be precise, the thyroid gland is a butterfly-shaped gland located in the middle of the neck, on the ventrolateral side of the trachea. It is a bi-lobed gland; that is, it contains two lobes connected by a non-glandular flap of connective tissue called the isthmus.

Structure of the thyroid gland

The thyroid gland is filled with a loose connective tissue called the stroma. This stroma contains numerous cuboidal cells that exist in groups called thyroid follicles. The function of the thyroid follicles is to synthesize thyroid hormones. Once produced, the thyroid hormone lies surrounded by the follicular cells inside the thyroid gland. This thyroid hormone is stored here for 90 days until our body needs it.

Apart from the follicular cells, the stroma contains some other cells, which are called C cells. The C cells are found in the interstitial tissue between the follicles. Their function is to release calcitonin, which is responsible for maintaining calcium levels in our bodies. For more details, refer to NCERT Solutions for Class 11 Biology.

Formation of the thyroid hormones

The thyroid gland synthesizes two types of thyroid hormones- T3 and T4. This happens in several steps as follows:

• Through active transport, the thyroid gland takes up all the iodine present in our blood vessels. The cuboidal follicular cells of the thyroid gland store this iodine and release a protein called thyroglobulin. 
• Thyroglobulin is filled inside the cavity of follicular cells. Being a protein, this thyroglobulin is mainly made up of a specific amino acid called tyrosine. This tyrosine binds with iodine to give rise to thyroid hormones. Therefore, tyrosine is the precursor amino acid for the formation of thyroid hormone. This point is very important for NEET-UG 2022.
•   If one molecule of iodine binds with tyrosine, it’s called monoiodotyrosine, and if two molecules of iodine combine with tyrosine, the end product is called di-iodotyrosine. If one mono-iodotyrosine and one di-iodotyrosine are combined, the result is the formation of triiodothyronine. This triiodothyronine is our thyroid hormone or T3 or thyroid hormone. If two di-iodotyrosines combine, the result is the formation of tetraiodothyronine. In this way, T3 and T4 are formed. More details regarding NEET 2022 are given here NCERT Solutions for Class 12
•  The remaining thyroglobulin gets digested, and only the iodinated tyrosines remain. 

The thyroid gland produces predominantly thyroxine (T4) -80% and only a small amount of T3 -20%. However, T3 is four times more effective than the T4 hormone.

Negative feedback mechanism of the thyroid hormones

When T3 and T4 (mainly T3) increase in the blood, a negative signal is given to the pituitary and hypothalamus. The pituitary gland is activated to decrease the production of TSH. Also, the hypothalamus is activated, stopping the secretion of TRH. This is the negative feedback mechanism caused by T3. This negative feedback mechanism helps maintain the levels of T3 and T4 in the body.

Functions of the thyroid gland/ thyroid hormones

1. The thyroid gland maintains the body’s basal metabolic rate (BMR). The basal metabolic rate indicates the rate of oxygen consumption by the body. Oxygen is used up in our body to produce ATP. So increased ATP is produced in our body because of the thyroid gland. This phenomenon is called the calorigenic effect of thyroid hormones.
2. All the cells of our body have a Na- K pump, which helps to extrude 3 Na+ out of the cell and take up 2 K+ inside the cell with the energy derived from the breakdown of ATP. Whenever ATP breaks down, energy in the form of heat is released. Thyroid hormones increase the functioning of Na K pumps, leading to increased ATP breakdown and increased production of heat. This phenomenon is called the thermogenic effect of the thyroid hormone.
3. The thyroid hormone is important for the body’s physical, mental, and reproductive growth. GH only causes physical growth. Deficiency of thyroid hormone can lead to stunted growth in children and irregular menstruation (hampered reproductive growth) in females. To support this growth, the thyroid hormone facilitates the formation of proteins or proteolysis.
4.  The thyroid hormones cause glycogen to break down in the liver and convert it into glucose. This process is called glycogenolysis. Also, they cause the breakdown of lipids (lipolysis) and the conversion of these lipids into glucose. This process is called gluconeogenesis. Therefore, the thyroid gland affects the metabolism of proteins, fats, and lipids.
5. The thyroid gland increases erythropoiesis or RBC production in the body. 
6. The thyroid gland maintains water and electrolyte balance in the body.
7. The thyroid gland regulates the menstrual cycle in females.

Disorders of the thyroid gland

The thyroid gland disorders are more common in females than in males.

Deficiency of thyroid hormone: Deficiency of thyroid hormone leads to various medical conditions such as myxoedema, cretinism and goitre. 

• Myxoedema: As discussed earlier, the thyroid hormone is important for the body’s physical, mental, and reproductive growth. Therefore, decreased production of thyroid hormone in myxoedema presents features like: 

1. Impaired sexual development
2. Infertility
3. Absent or under-developed secondary sexual characters
4. Amenorrhea or stopped menstruation

• Cretinism: “Cretin” means “idiot” or someone with a low IQ level. A baby growing inside a hypothyroid mother doesn’t receive an adequate amount of thyroid hormone from its pregnant mother. As a result, such a baby’s physical, mental, and sexual growth is not sufficiently mature. Such a condition is called cretinism. Following are the features of a baby with cretinism:

1. Low IQ or intelligence
2. Deaf and dumb
3. Stunted growth
4. Coarse and abnormal skin

• Goitre: It is the abnormal enlargement of the thyroid gland 
• Simple goitre: is also called a colloid goitre. Due to iodine deficiency in the diet, the formation of thyroid hormone is decreased. To compensate for this decreased function, the thyroid gland swells up. This condition frequently occurs in people living in mountainous regions. As it’s endemic to people living near the mountains, it is called endemic goitre.
• Deficiency of the thyroid gland leads to obesity.

Excess thyroid hormone: Excessive thyroid hormone in the body can lead to medical conditions such as Grave’s disease/ Myasthenia Gravis, Hashimoto’s thyroiditis, and exophthalmic goitre.

• Hashimoto’s thyroiditis: It is an autoimmune disorder. Normally, the hypothalamus releases TRH (thyrotropin-releasing hormone), whose function is to activate TSH release from the pituitary gland. To carry this out, the thyroid gland has certain receptors on its surface called thyroid receptors. When TSH released from the pituitary gland activates these thyroid receptors, the thyroid hormones- T3 and T4 are formed. Once the level of T3 and T4 rises high enough in the body, they inhibit the formation of more T3 and T4, which is called a negative feedback mechanism. However, in the case of Hashimoto’s thyroiditis, certain antibodies are present in the body which mimic the action of TSH. So when these autoantibodies combine with the TSH receptor, T3 and T4 formation occur, but the negative feedback mechanism fails to activate when their level increases. This causes T3 and T4 to increase to a hazardous level, leading to the formation of an array of symptoms like:

1. Increased weight loss  
2. Protruding eyeballs
3. Increased activation of the adrenaline gland/ increased formation of adrenal hormone: which causes symptoms like fight or flight reaction, increased sweating, etc.

Fun facts about the thyroid gland

1. The thyroid gland is the only endocrine gland of our body that stores its secretions for around 90-100 days.
2. The thyroid gland is the largest endocrine gland of the body.
3. Thyroid cells are the only cells of our body specialized in taking up iodine and converting it into hormones. Check NEET 2022 Important Chapters

Such questions are important from the point of view of the NEET 2022 Exam Pattern, NEET UG 2022. The topic of the thyroid gland is very important in the NEET 2022 Syllabus. 

We now know that the thyroid gland is an endocrine gland that is very important for the normal functioning of our bodies. It is under the strict control of the pituitary gland and the hypothalamus. It secretes two types of hormones- T3 or tri-iodothyronine, T4, or thyroid. The release of thyroid hormones in our body needs to be regulated. Both their excess and deficiency lead to the appearance of symptoms and cause various complications in our body. 

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