Thyroid Physiology 2

A protein found in the colloid of the thyroid gland.

The process of adding an iodine atom to tyrosine.

A transporter responsible for moving iodide from the blood into the thyroid follicular cells.

A hormone produced by the hypothalamus that travels to the pituitary gland.

A hormone produced by the pituitary gland that stimulates the thyroid gland.

Iodine is taken up by the thyroid gland and converted to iodide. Iodide then travels through the bloodstream.

The sodium-iodide symporter (NIS) is located on the basal membrane of follicular cells. This protein pumps iodide from the blood into the follicular cell.

Thyroglobulin is a glycoprotein made in the follicular cells. It is synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, and released into the colloid via exocytosis.

Thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) are synthesized and stored in the colloid of the thyroid gland.

The release of T3 and T4 from the thyroid gland is regulated by the pituitary gland, via the hormone TSH (thyroid stimulating hormone).

T3 increases mental alertness and shortens nerve reaction times, leading to increased responsiveness to stimuli.

T3 increases appetite, GI motility, secretion, and glucose absorption.

T3 mobilizes free fatty acids, reducing fat storage, decreases plasma cholesterol by increasing biliary excretion, and lowers plasma triglycerides.

T3 helps maintain balanced protein synthesis and breakdown, affects muscle contraction strength and relaxation time, influences energy utilization, and assists with muscle repair after injury.

T3 increases sensitivity of heart muscle cells to catecholamines like epinephrine and norepinephrine by increasing adrenergic receptor expression.

T3 causes vasodilation in skin for heat dissipation and in heart and muscle to increase blood flow.

T3 increases TSH production, influencing growth hormone secretion and playing a key role in fetal and neonatal development, including brain maturation, axonal growth, myelin sheath formation, muscle development, and bone growth.

Euthyroid sick syndrome is a condition where thyroid hormone concentrations are suppressed due to illness, stress, trauma, or surgery. TT4 is particularly affected, while fT4 is suppressed in moderate to severe illness. TSH levels are typically normal.

A protein in the blood that binds to thyroid hormones (T3 and T4), transporting them throughout the body.

A protein in the blood that binds to thyroid hormones (T3 and T4), transporting them throughout the body.

A decrease in the amount of binding proteins in the blood, leading to lower measured T3 and T4 levels, but higher free T3 and T4 levels.

A thyroid hormone that directly impacts the metabolism of cells.

A thyroid hormone that is converted into its active form (T3) inside cells.

The receptor within the nucleus of cells that binds to thyroid hormones, triggering gene expression.

The process by which thyroid hormones bind to the thyroid hormone receptor (TR) and regulate gene transcription, affecting everything from metabolism to hair growth.

The overall metabolic effects of T3, including increased tissue metabolism, heat production, energy usage and a balance in protein synthesis and breakdown.

Thyroid hormone (T4) travels in the blood mainly bound to proteins. About 99% is bound to proteins, while only 1% is free and bioavailable.

Thyroxine-binding globulin (TBG) is a protein in the blood that binds thyroid hormones, primarily T4. It has a higher affinity for T4 than T3.

T3 is the most biologically active form of thyroid hormone, meaning it directly influences cellular processes.

T3 increases target cellular activity by binding to intracellular receptors, triggering gene transcription and protein synthesis.

Thyroid hormones increase basal metabolic rate, the rate at which the body burns calories at rest.

Thyroid hormones regulate nervous system function, influencing alertness, mood, and cognitive abilities.

Thyroid hormones stimulate digestion and nutrient absorption in the gastrointestinal tract.

Thyroid hormones increase heart rate and contractility, leading to improved blood circulation.

Thyroid hormones promote filtration of blood in the kidneys, increasing glomerular filtration rate (GFR).

Thyroid hormones are essential for normal fetal and juvenile growth and development.

Thyroid hormone synthesis requires two essential components: tyrosine, an amino acid found in thyroglobulin, a large protein stored in the colloid of thyroid follicles, and iodine, which is obtained from dietary iodide.

Thyroglobulin (TG) is a large protein found within the colloid of the thyroid gland. It plays a vital role in thyroid hormone synthesis by providing tyrosine residues, which are essential for the creation of thyroid hormones.

Iodine, in the form of iodide (I-), is obtained from the diet and is essential for thyroid hormone synthesis. It is transported into the thyroid gland and eventually incorporated into thyroglobulin to form thyroid hormones.

The thyroid gland is a butterfly-shaped gland located in the front of the neck. It is responsible for producing and secreting thyroid hormones, such as T3 and T4, which play essential roles in regulating metabolism, growth, and development.

The colloid is a gel-like substance found within the thyroid follicles. It contains thyroglobulin, a large protein that serves as a storage depot for thyroid hormone precursors.

The follicular cells are specialized cells that line the thyroid follicles. They play a key role in thyroid hormone synthesis by producing and secreting thyroglobulin, transporting iodide, and converting thyroid hormone precursors into active hormones.

The sodium-iodide symporter (NIS) is a protein embedded in the follicular cell membrane. It actively transports iodide from the blood into the follicular cells, making it available for thyroid hormone synthesis.

Pendrin is a protein that transports iodide from the follicular cells into the colloid. This step is crucial for ensuring adequate iodide levels in the colloid for thyroid hormone synthesis.

Thyroid peroxidase (TPO) is an enzyme that plays multiple roles in thyroid hormone synthesis, including the oxidation of iodide, the iodination of tyrosyl residues on thyroglobulin, and the coupling of iodinated tyrosine residues to form thyroid hormones.

Organification refers to the process where iodine is added to tyrosine residues on thyroglobulin. This step is crucial for producing thyroid hormone precursors, MIT and DIT.