Thyroid gland and its regulation

Overview of thyroid

Each of our body systems is under the control of one or more other systems directly or indirectly. One of the most dominant systems that can control other body systems is the nervous system followed by the endocrinal system. The endocrinal system has a number of glands among which thyroid is very essential. The thyroid gland synthesizes the hormones thyroxine in 2 variants; thyroxine (T4) and triiodothyronine (T3), containing iodine. Their fundamental function is to regulate the body's metabolic rate hence they are generally called metabolic hormones. They must be synthesized in an adequate amount to achieve the normal growth and development of the nervous system in children. Furthermore, it helps to maintain normal skeletal growth and maturation during different phases of growth. Adults need thyroxine for many of the body functions hence it has an influence on multiple-organ systems.   Let us discuss the normal structure, functions and mechanism related to thyroxine in our body

 

 

Anatomy of thyroid

The normal thyroid gland is firm, smooth and reddish-brown in its appearance.  An adult`s thyroid hormone contains 2 lateral lobes and a connecting central isthmus; a stalk-like structure that holds the gland in place.  Approximate weight of the normal thyroid ranges from 30-40 grams, however, thyroid diseases can result in excess growth and weight of the gland.  On its outer wall, it is enclosed by a capsule from which a number of fibrous projections pop out and these projections are re-divided into many tiny lobules. The thyroid gland has a rich blood supply and it is one of the organs requiring the highest rates of blood flow per gram of tissue when compared to any other organ.

 

 

 

 

 

 

 

 

Microanatomy

The thyroid gland contains many closely packed acini, called follicles. Each of these is surrounded by capillaries and stroma. An individual follicle is roughly spherical, lined by a single layer of cuboidal epithelial cells filled with proteinaceous material called colloid. A colloid is like a ground substance mixed with thyroglobulin and thyroid hormones in it. Alongside the follicles, the parafollicular cells secrete calcitonin- a hormone that protects the bones by regulating the plasma calcium level. The follicular cell accommodates on a basal lamina that contains round and centrally located nucleus. Follicular cells have the cytoplasm containing mitochondria, rough endoplasmic reticulum, and ribosomes. They are the most active and most primary secretory cells of the thyroid gland.

 

 

Physiology of thyroid

  1. The thyroid gland as a whole performs a number of duties among which follicular cells play an important role. The cells offer 3 major functions; I,e  collecting and transporting the iodine to the colloid, synthesizing thyroglobulin, and the release of thyroid hormones from thyroglobulin into the circulation.

  2. They also assist in iodine Metabolism and Trapping. An adult requires a minimum of 150 µg of iodine per day. The follicular cells carry the iodide from the circulation into the colloid of the gland where it is trapped and safeguarded.

  3. Thyroid regulates bone and muscle mass by improving the necessary mineral metabolism in the body.

  4. T3-triiodothyronin acts on our body in many ways. The T3 help to increase cardiac output, heart rate, ventilation rate and the basal metabolic rate.

  5. They potentiate brain development in children hence they are very essential for the brain development of children.

  6. The thyroid also helps in thickening of inner uterine layer-endometrium in females thus making fetus safe.

  7. It is one of the most vital hormones to increase the catabolism of proteins and carbohydrates.

 

 

 Thyroid secretions

The apical cell membrane of the follicular cells produces the thyroid hormone. The enzyme thyroidal peroxidase help in oxidizing the iodide into iodine. The thyroid secretes about 103 mol of T4 and 7 nmol of T3 per day. After the production, the free T4 and T3 cross the cell membrane to reach the place where it stores. T4 level in our blood is approximately 103 nmol/and the T3 level is roughly around 2.3 nmol/L. Both of them are bound to plasma proteins such as albumin, transthyretin and thyroxine-binding globulin. These thyroid hormone-binding proteins help to transport T4 and T3 in the serum and to facilitate uniform distribution of hormones across tissues. T4 and T3 are metabolized in the liver, kidneys, and many other tissues and it takes place with the help of the processes called denomination and conjugation of glucuronides.

 

 

Regulation of thyroid secretion

Thyroid-stimulating hormone (TSH, thyrotropin) is one of the most important glands that regulate the thyroid. Our pituitary gland secrets TSH which in turn, stimulates the thyroid follicles to act quickly. Additionally, thyrotropin-releasing hormone (TRH) secreted by the hypothalamus increases the biologic activity of TSH, by altering its glycosylation.

 

Target Tissue

Effect

Mechanism

Heart

Chronotropic

  • Increase number and affinity of β-adrenergic receptors.

Inotropic

  • Enhance responses to circulating catecholamines.

  • Increase the proportion of alpha-myosin heavy chain (with higher ATPase activity).

Adipose tissue

Catabolic

  • Stimulate lipolysis.

Muscle

Catabolic

  • Increase in protein breakdown.

Bone

Developmental and metabolic

  • Promote normal growth and skeletal development; accelerate bone turnover.

Nervous system

Developmental

  • Promote normal brain development.

Gut

Metabolic

  • Increase the rate of carbohydrate absorption.

Lipoprotein

Metabolic

  • Stimulate the formation of LDL receptors.

Other

Calorigenic

  • Stimulate oxygen consumption by metabolically active tissues (exceptions: adult brain, testes, uterus, lymph nodes, spleen, anterior pituitary).

 

 

 

Mechanism of Action of  thyroid hormones

Thyroid hormones enter cells by either passive diffusion or specific transport through the cell membrane and cytoplasm. Within the cell cytoplasm, most of the T4 is converted to T3. The nuclear receptor for T3 has been cloned and found to be similar to the nuclear receptors for glucocorticoids, mineralocorticoids, estrogens, progestin, vitamin D3, and retinoic acid.

 

 

Read more

1.    

Endocrinal control and coordination

2.

Pituitary gland and pituitary hormones

3.

SIADH

4.        

Adrenal glands

 

 

 

Questions

  1. What is the principal function of thyroid hormone?

  2. Where is thyroid located?

  3. Explain the anatomy of the thyroid gland

  4. Name the hormones secreted by thyroid gland and mention at least one function of each.

  5. What is TSH, explain its role in thyroid function.

  6. Brief the mechanism of action of thyroid hormones

 



img-1


img-1


img-1

Course List