Thyroid gland

What is the storage form of thyroid hormones?

The storage form of thyroid hormones is thyroglobulin.

Formation, Storage, and Release of Thyroid Hormones

The thyroid gland is the only endocrine gland that stores its secretory product in large quantities—normally about a 100‐day supply.

Thyroid Gland Hormones: T3 (triiodothyronine) and T4 (thyroxine) or thyroid hormones from follicular cells

Control of secretion: Secretion is increased by thyrotropin‐releasing hormone (TRH), which stimulates release of thyroid‐stimulating hormone (TSH) in response to low thyroid hormone levels, low metabolic rate, cold, pregnancy, and high altitudes. -TRH and TSH secretions are inhibited in response to high thyroid hormone levels; high iodine level suppresses secretion. Principle Actions: -Increase basal metabolic rate; stimulate synthesis of proteins; increase use of glucose and fatty acids for ATP production; increase lipolysis; enhance cholesterol excretion; accelerate body growth; contribute to development of nervous system.

Which cells secrete T3 and T4 ? Which secrete calcitonin? Which of these hormones are also called thyroid hormones?

Follicular cells secrete and , also known as thyroid hormones. Parafollicular cells secrete calcitonin.

How could an iodine‐deficient diet lead to goiter, which is an enlargement of the thyroid gland?

Lack of iodine in the diet → diminished production of T3 and T4 → increased release of TSH → growth (enlargement) of the thyroid gland → goiter.

Thyroid follicle

Spherical sac that forms the parenchyma of the thyroid gland and consists of follicular cells that produce thyroxine (T4) and triiodothyronine (T3).

review

Thyroid hormones are synthesized by attaching iodine atoms to the amino acid tyrosine.

Actions of Thyroid Hormones: 1. basal metabolic rate

Thyroid hormones increase basal metabolic rate (BMR), the rate of oxygen consumption under standard or basal conditions.

Control of Thyroid Hormone Secretion

Thyrotropin‐releasing hormone (TRH) from the hypothalamus and thyroid‐stimulating hormone (TSH) from the anterior pituitary stimulate synthesis and release of thyroid hormones.

Thyroid Gland Hormones: Calcitonin (CT) from parafollicular cells

control of secretion: -High blood levels stimulate secretion; low blood levels inhibit secretion. Principle Actions: Lowers blood levels of calcium and HPO by inhibiting bone resorption by osteoclasts and by accelerating uptake of calcium and phosphates into bone extracellular matrix.

basement membrane

surrounds each follicle.

thyroid gland

-An endocrine gland with right and left lateral lobes on either side of the trachea connected by an isthmus. -located anterior to the trachea just inferior to the cricoid cartilage.

Synthesis and secretion of T3 and T4: 4. Iodination of tyrosine.

-As iodine molecules form, they react with tyrosines that are part of thyroglobulin molecules. -Binding of one iodine atom yields monoiodotyrosine (T3), and a second iodination produces diiodotyrosine (T4). -The TGB with attached iodine atoms, a sticky material that accumulates and is stored in the lumen of the thyroid follicle, is termed colloid.

Synthesis and secretion of T3 and T4: 7. Secretion of thyroid hormones.

-Because T3 and T4 are lipid‐soluble, they diffuse through the plasma membrane into interstitial fluid and then into the blood.

follicular cells

-Cells stimulated to develop alongside the oocyte. -secrete the thyroid hormones.

Synthesis and secretion of T3 and T4: 6. Pinocytosis and digestion of colloid.

-Droplets of colloid reenter follicular cells by pinocytosis and merge with lysosomes. Digestive enzymes in the lysosomes break down TGB, cleaving off molecules of T3 and T4.

Synthesis and secretion of T3 and T4: 5. Coupling of T1 and T2 .

-During the last step in the synthesis of thyroid hormone, two T2 molecules join to form T4, or one T1 and one T2 join to form a T3 .

Actions of Thyroid Hormones: 3. metabolism

-In the regulation of metabolism. -the thyroid hormones stimulate protein synthesis and increase the use of glucose and fatty acids for ATP production.

Synthesis and secretion of T3 and T4: 8. Transport in the blood.

-More than 99% of both the T3 and the T4 combine with transport proteins in the blood, mainly thyroxine‐binding globulin (TBG).

Synthesis and secretion of T3 and T4: 3. Oxidation of iodide.

-Some of the amino acids in TGB are tyrosines that will become iodinated. -negatively charged iodide ions cannot bind to tyrosine until they undergo oxidation (removal of electrons). -As the iodide ions are being oxidized, they pass through the membrane into the lumen of the follicle.

Synthesis and secretion of T3 and T4: 1. Iodide trapping.

-Thyroid follicular cells trap iodide ions by actively transporting them from the blood into the cytosol. As a result, the thyroid gland normally contains most of the iodide in the body

Actions of Thyroid Hormones: 5. body growth

-Together with human growth hormone and insulin, thyroid hormones accelerate body growth, particularly the growth of the nervous and skeletal systems.

Synthesis and secretion of T3 and T4: 2. synthesis of thyroglobulin.

-While the follicular cells are trapping iodine ions, they are also synthesizing thyroglobulin), a large glycoprotein that is produced in the rough endoplasmic reticulum, modified in the Golgi complex, and packaged into secretory vesicles. -The vesicles then undergo exocytosis, which releases TGB into the lumen of the follicle.

Actions of thyroid hormones:

-increase basal metabolic rate. -stimulate synthesis of sodium and potassium. -increase body temp. -stimulate protein synthesis. -increase the use of glucose and fatty acids to ATP. -stimulate lypolysis. -enhance some actions of catecholamines. -regulate development and growth of nervous tissue and bones.

what hormones does the thyroid gland secrete?

-secretes thyroxine (T4), triiodothyronine (T3), and calcitonin.

Actions of Thyroid Hormones: 4. enhance actions of catecholamines

-thyroid hormones enhance some actions of the catecholamines (norepinephrine and epinephrine) because they up‐regulate beta (β) receptors.

Actions of Thyroid Hormones: 2. additional sodium-potassium pumps

-thyroid hormones stimulate synthesis of additional sodium-potassium pumps ( ATPase). -which use large amounts of ATP to continually eject sodium ions from the cytosol into the extracellular fluid and potassium ions from the extracellular fluid into the cytosol.

what do thyroid hormones regulate?

1. oxygen use and basal metabolic rate. 2. cellular metabolism. 3. growth and development.

Actions of Thyroid Hormones

Because most body cells have receptors for thyroid hormones, T3 and T4 exert their effects throughout the body.