where is the thyroid gland located
located over the trachea just below the larynx
what does the thyroid gland consist of
- it consists of two lobes connected by a thinner section of the gland, known as the isthmus
- there is no difference between the lobes
- the entire gland serves the same function, that is to produce and secrete thyroid hormones
explain the cellular structure of the thyroid gland
- follicular cells are arranged to form hollow spheres throughout the gland
- when looking at a microscopic section, the spheres/follicles appear as ring-like structures
- within the thyroid gland are another cell type called C cells that secrete calcitonin
what is the colloid made up of
primarily made up of a large protein molecule called thyroglobulin, which is where the thyroid hormones are synthesized and stored
explain thyroid hormones
- the thyroid gland produces 2 hormones from the aa tyrosine
- both thyroid hormones contain iodine
- both hormones are referred to as thyroid hormones as they exert the same physiological effects, only with differences in their speed and intensity of action
what are the 2 types of thyroid hormones
- T3: considered to be more active thyroid hormone
- T4: converted to T3 in target tissues
explain Tetraiodothyronine (T4)
this hormone contains 4 iodine molecules and represents about 90% of the thyroid hormones secreted
explain Triiodothyronine (T3)
this hormone contains 3 iodine molecules and represents the remaining 10% if secreted hormone
explain the importance of iodine
- the follicular cells obtain the basic building blocks of tyrosine and iodine from the blood
- the body requires about 1 mg of iodine a week to ensure sufficient levels of thyroid hormone
- once ingested, iodine circulates as iodide (I.)
explain the thyroid gland with iodine
- the thyroid gland is very efficient in extracting iodine from the blood such that essentially all iodine is taken up into the gland
- the thyroid gland stores enough thyroid hormone to endure long periods when dietary iodine is not available
how do Canadians meet their dietary need of iodine
the consumption of table salt, which is supplemented with iodine
why are thyroid hormones an exception to lipophilic hormones
- lipophilic hormones are not stored but are released upon their synthesis
- fully formed thyroid hormones can be protected from secretion while they are stored bound to thyroglobulin in the colloid of the thyroid gland
what is the first step of the synthesis of thyroid hormones
tyrosine-containing thyroglobulin is produced within the follicular cells by the endoplasmic reticulum-Golgi complex and is transported to the colloid by exocytosis
what is the second step of the synthesis of thyroid hormones
- iodide is taken up by follicular cells through a process called iodide trapping
- iodide is driven against its concentration gradient by using a Na+ cotransporter that moves Na+ down its concentration gradient
what is the third step of the synthesis of thyroid hormones
iodide is then transferred into the colloid of the follicular lumen
what is the fourth step of the synthesis of thyroid hormones
- almost simultaneous to the iodide moving into the colloid, the enzyme thyroperoxidase converts iodide into a highly reactive state, iodine (Io), which immediately attaches to a tyrosine residue on a thyroglobulin molecule
- this process is called iodide organification
- the attached of one iodine to a tyrosine produces monoiodotyrosine (MIT), while the attachement of a second iodine produced diiodotyrosine (DIT)
what is the sixth step of the synthesis of thyroid hormones
- a coupling process occurs that combines MITs and DITs to form the thyroid hormones
- the coupling of one MIT and one DIT forms triiodothyronine (T3) while the coupling of two DITs forms tetraiodothyronine (T4)
- there is no coupling of two MITs
( both T3 and T4 remain bound to the thyroglobulin molecule after the chemical reactions)
what is the first step of the release of thyroid hormones
the follicular cells engulf a portion of the thyroglobulin-containing colloid by phagocytosis and create hormone-filled vesicles
what is the second step of the release of thyroid hormones
once inside the follicular cell, lysosomes fuse with the vesicles and digestive enzymes release all of the MIT, DIT, T3, and T4 from the thyroglobulin
what is the third step of the release of thyroid hormones
because T3 and T4 are very lipophilic they immediately cross the plasma membrane to the blood where they bind to plasma proteins, mainly thyroid-binding globulin
explain the actions of thyroid hormones
- thyroid hormones have effects on most tissues of the body, either directly or indirectly
- the actions of thyroid hormones are slow compared to other hormones
- it can take hours before the effects are observed and the duration can last for days even after plasma concentrations of thyroid hormone have returned to normal
explain the major effect of thyroid hormones - metabolic rate and heat production
- thyroid hormone increases the overall basal metabolic rate by increasing oxygen consumption and energy expenditure
- a consequence of increasing metabolic rate is an increased heat production
explain the major effect of thyroid hormones - intermediary metabolism
thyroid hormone influences the enzymes involved in fuel metabolism, but the effects differ depending on the amount of thyroid hormone present
what is an example of intermediary metabolism
at low concentrations of thyroid hormone, the conversion of glucose to glycogen and protein synthesis are favoured, whereas at higher concentrations, thyroid hormone causes the breakdown of glycogen to glucose and the degradation of proteins
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