Changes

Follicular cells synthesize '''Thyroglobulin''' in their golgi apparatus. This is a glycoprotein consisting of 70 linked tyrosine molecules, 10% of which are ''iodinated'', and is stored in the colloid.  

Follicular cells synthesize '''Thyroglobulin''' in their golgi apparatus. This is a glycoprotein consisting of 70 linked tyrosine molecules, 10% of which are ''iodinated'', and is stored in the colloid.  

The thyroglobulin is then split  to form the two amino acid derivative [[Hormones - Anatomy & Physiology|hormones]] produced in the thyroid gland which are Triiodothyronine (T3) and Thyroxine (T4).

The thyroglobulin is then split  to form the two amino acid derivative [[Hormones - Anatomy & Physiology|hormones]] produced in the thyroid gland which are Triiodothyronine (T3) and Thyroxine (T4). Thyroxine contains 4 iodine atoms, triiodothyronine contains 3. Creation of these two [[Hormones - Anatomy & Physiology|hormones]] is the only role of iodine in the body.

 

Thyroxine contains 4 iodine atoms, triiodothyronine contains 3. Creation of these two [[Hormones - Anatomy & Physiology|hormones]] is the only role of iodine in the body.

 

The majority (90%) of hormone produced by the follicular cells is T4. T4 can only be made in the thyroid gland. It can then be converted by other tissues into T3.

The majority (90%) of hormone produced by the follicular cells is T4. T4 can only be made in the thyroid gland. It can then be converted by other tissues into T3.

===Thyroid Hormone Actions===

===Thyroid Hormone Actions===

T3 and T4 have effects on all body systems and at all stages of life:

T3 and T4 have effects on all body systems and at all stages of life. These include '''Development''' where thyroid hormones are vital during the fetal period and the first few months after birth. T3 and T4 are the hormones for metamorphosis in frogs. Thyroid hormones also promote '''Growth''' as they enhance amino acid uptake by tissues and enzymatic systems involved in protein syntheis, promotes bone growth. They also help with metabolic actions such as '''Carbohydrate metabolism''', as thyroid hormones stimulate glucose uptake, glycogenolysis, gluconeogenesis. In '''Fat metabolism''' they mobilise lipids from adipose stores and accelerate oxidation of lipids to produce energy (occurs within mitochondria), as well as increasing the size and number of mitochondria. Thyroid hormones also '''Increase Basal Metabolic Rate''' (BMR) in all tissues except brain, spleen and gonads. The results in increased heat production, increased oxygen consumption. This increased metabolic rate also results in increased utilisation of energy substrates causing weight loss. Some of thyroid hormones [[Cardiorespiratory System Overview - Anatomy & Physiology#Cardiovascular System|'''Cardiovascular actions''']] are to increase cardiac output, heart rate and contractility. They affect the [[Cardiorespiratory System Overview - Anatomy & Physiology#Upper Respiratory Tract|'''Respiratory system''']] indirectly through increased BMR causing increased demand for oxygen and increased excretion of carbon dioxide. In the [[Nervous and Special Senses - Anatomy & Physiology#Nervous System|'''Nervous system''']] thyroid hormones are required for myelination of [[PNS Structure - Anatomy & Physiology#Neurons|neurons]] during the development of this system. They also enhance the sympathetic nervous system (by increasing epinephrine receptors). [[Reproductive System Overview - Anatomy & Physiology|'''Reproductive System''']] is affected if thyroid hormone levels decrease reduced levels of thyroid hormone causes irregular cycling, decreased libido. Finally, in the [[Alimentary System Overview - Anatomy & Physiology|'''Alimentary System''']] - Thyroid hormone increases appetite and feed intake, increases secretion of [[Pancreas - Anatomy & Physiology|pancreatic]] enzymes and increases motility.

 

*'''Development''' - Thyroid hormones are vital during the fetal period and the first few months after birth. T3 and T4 are the hormones for metamorphosis in frogs.

*'''Growth''' - enhances amino acid uptake by tissues and enzymatic systems involved in protein syntheis, promotes bone growth

**'''Carbohydrate metabolism''' - stimulates glucose uptake, glycogenolysis, gluconeogenesis

**'''Fat metabolism''' - Mobilises lipids from adipose stores. Accelerates oxidation of lipids to produce energy (occurs within mitochondria). Increases size and number of mitochondria.

**'''Increase Basal Metabolic Rate''' (BMR) - in all tissues except brain, spleen, gonads. Results in increased heat production, increased oxygen consumption.

**'''Decrease body weight''' - Increased BMR results in increased utilisation of energy substrates

*[[Cardiorespiratory System Overview - Anatomy & Physiology#Cardiovascular System|'''Cardiovascular actions''']] - increases Cardiac output, heart rate and contractility.

*[[Cardiorespiratory System Overview - Anatomy & Physiology#Upper Respiratory Tract|'''Respiratory system''']] - indirectly affected through increased BMR causing increased demand for oxygen and increased excretion of carbon dioxide.

*[[Nervous and Special Senses - Anatomy & Physiology#Nervous System|'''Nervous system''']] - thyroid hormones are required for myelination of [[PNS Structure - Anatomy & Physiology#Neurons|neurons]] during the development of this system. They also enhance the sympathetic nervous system (by increasing epinephrine receptors).

*[[Reproductive System Overview - Anatomy & Physiology|'''Reproductive System''']] - reduced levels of thyroid hormone causes irregular cycling, decreased libido.

*[[Alimentary System Overview - Anatomy & Physiology|'''Alimentary System''']] - Thyroid hormone increases appetite and feed intake, increases secretion of [[Pancreas - Anatomy & Physiology|pancreatic]] enzymes and increases motility.

==Problems associated with the Thyroid Gland==

==Problems associated with the Thyroid Gland==

 

Problems with the thyroid gland include enlargement, or [[Goitre|Goitre]] and also the effects of increased level of hormones in [[Hyperthyroidism|Hyperthyroidism]] or decreased levels of hormones in [[Hypothyroidism|Hypothyroidism]].

*[[Goitre|Goitre]]

*[[Hyperthyroidism|Hyperthyroidism]]

*[[Hypothyroidism|Hypothyroidism]]

 

==Functional Anatomy (summary)==

==Functional Anatomy (summary)==

The thyroid gland lies in the neck, in front of the upper part of the trachea.  Two types of hormones are produced:

The thyroid gland lies in the neck, in front of the upper part of the trachea.  Two types of hormones are produced, which are the iodine containing hormones; '''Tri-iodothyronine'''(T3) and '''Thyroxine''' (T4).  Thyroid hormones regulate the basal metabolic rate and are important in the regulation of growth of tissues, particularly nervous tissue.  Release stimulated by TSH from the pituitary. The second type of hormone produced from the thyroid gland is '''Calcitonin''', which regulates blood calcium levels along with parathyroid hormone and acts to reduce blood calcium by inhibiting its removal from bone.   

*Iodine-containing hormones: '''Tri-iodothyronine'''(T3) and '''Thyroxine''' (T4).  Thyroid hormones regulate the basal metabolic rate and are important in the regulation of growth of tissues, particularly nervous tissue.  Release stimulated by TSH from the pituitary.

*'''Calcitonin''': Regulates blood calcium levels along with parathyroid hormone.  Acts to reduce blood calcium by inhibiting its removal from bone.   

The majority of the gland is derived from a downgrowth of the foetal tongue.  The calcitonin producing cells are different and are derived from the fourth branchial pouch.   

The majority of the gland is derived from a downgrowth of the foetal tongue.  The calcitonin producing cells are different and are derived from the fourth branchial pouch.   

The throid gland is divided into follicles which are bounded by a single layer of cuboidal epithelial cells and a basement membrane.  Follicles contain a homogenous colloid material called '''thyroglobulin'''.  This is a store of thyroid hormones prior to secretion.  The thyroid gland is the only endocrine gland to store its hormone in large quantities.   

The throid gland is divided into follicles which are bounded by a single layer of cuboidal epithelial cells and a basement membrane.  Follicles contain a homogenous colloid material called '''thyroglobulin'''.  This is a store of thyroid hormones prior to secretion.  The thyroid gland is the only endocrine gland to store its hormone in large quantities.   

In the active gland:

In the active gland colloid is diminished and epithelial cells are tall and columnar.   

*Colloid is diminished.

*Epithelial cells are tall and columnar.   

'''Parafollicular''' cells are found in clusters in the interfollicular space.  Also known as '''clear''' cells as their cytoplasm doesn't stain with H and E.

'''Parafollicular''' cells are found in clusters in the interfollicular space and are also known as '''clear''' cells as their cytoplasm doesn't stain with H and E.

These cells synthesise and secrete calcitonin in response to raised plasma calcium.

These cells synthesise and secrete calcitonin in response to raised plasma calcium.