The Thyroid Gland Flashcards
What is secreted by the thyroid gland?
- T4: thyroxine
- T3: triiodothyronine
- calcitonin
- thyroglobulin (into follicular sacs within the gland)
Secretion is regulated by TSH secreted by the anterior pituitary
Describe the compartments and secretions of the thyroid gland
- the thyroid gland contains closed follicles
- cuboidal cells line it and secrete colloid containing thyroglobulin (contains the hormones) into the follicles
- secretions are reabsorbed by the epithelial cells and enter bloodstream before exerting its function in the body
- C cells (located sporadically on exterior surface of epithelial cells lining follicles) secrete calcitonin
Describe the secretion and then function of T3 and T4
- most of active hormones released are T4 but almost all of it will be converted to T3 in the tissues through de-iodination
- T3 is more potent, contained in smaller quantities and short life span
- they increase metabolic rate (through transcription and non-genomic cellular effects) - so increase in secretion = weight loss and vice versa
What happens during abnormal rate of T3 and T4 secretion?
Lack of secretion causes basal metabolic rate to fall 40-50% below normal
Excessive secretion can increase metabolic rate to 60-100% above normal
Describe the role of iodine
- needed for formation of thyroxine (combines with tyrosine within thyroglobulin molecule to form T3 and T4)
- taken up by the thyroid gland
- excess can be secreted by the kidneys
Describe the process of iodide trapping
- transport of iodine from the blood to the thyroid gland cells and follicles
- basolateral membrane of thyroid cell actively pumps iodide into cells by sodium-iodide symporter (2Na+: 1 iodide) against its concentration gradient
- iodide concentrated in cell (up to 30-250x concentration in plasma)
- rate is controlled by TSH
- iodide then transported through apical membrane into follicle by ‘pendrin’ (chloride-iodide antiporter)
Describe the process of iodide conversion to iodine (and the clinical significance)
- iodide ions are converted to oxidised iodine to be combined with tyrosine
- enzyme peroxidase which is located/attached to the apical membrane produces peroxide and this oxidises the iodide ions
- blockage/absence of perioxidase system stops formation of thyroid hormones
Describe the process of iodination and coupling
- tyrosine combines with iodine to form monoiodotyrosine and then again to diiodotyrosine
- over time, the mono and/or diiodotyrosine residues become coupled with one another
- 2 diiodotyrosine = T4
- mono + diiodotyrosine = T3
- di + monoiodotyrosine = reverse T3 (non-functional)
Describe how hormones are released from the thyroid gland
- T3 and T4 can be cleaved from thyroglobulin and released as free hormones
- colloid (containing thyroglobulin with T3 and T4) are brought into the cell at its apex through pinocytosis
- once in, vesicles fuse with lysosomes containing protease which digests thyroglobulin and releases T3 and T4
- hormones can then diffuse through basolateral membrane into capillaries and bloodstream to be delivered to tissues
How are the T3 and T4 hormones transported in the blood?
- bound to proteins: thyroxine-binding globulin, thyroxine-binding prealbumin and albumin
- proteins also bind (due to high affinity) and so only release the hormones slowly at tissues (T3 released quicker than T4)
Describe the action of T3 and T4 in transcription of genes
- T3 and T4 enter the cell membrane of the target cell through carrier-mediated active transport
- most T4 deionated (through deiodinase) to from T3 which then interacts with thyroid hormone receptor
- leads to either increase or decrease in transcription of genes to form proteins
- response of the cell is generated
Describe the effects of thyroid hormones on nutrient absorption and storage
- increase glucose uptake in GIT
- increase in glucose utilisation esp in muscle and adipose tissue
- increase liver glycogenolysis and gluconeogenesis
- increase lipolysis in adipose tissue (increase in plasma free fatty acids)
- increase tissue oxidation of free fatty acids
- increase in protein turnover and anabolism
- allows action of other hormones (GH, prolactin, gonadal and adrenal steroids)
- maintains normal development and function of nervous systems
List the major physiological effects of thyroid hormones
- increase in blood flow and CO
- increase in respiration
- increase in GI motility
- increase in excitatory effects of CNS (worry, anxiety etc)
- increased rate of secretion of other hormones
- increase muscle vigour (tremor) that decreases once it hits high concentrations
- sexual dysfunction
Describe the regulatory process of T3 and T4 secretion through TSH
- TSH increases T3 and T4 secretion mediated by cAMP
- secretion of TSH is controlled by a hypothalamic hormone TRH (thyrotropin-releasing-hormone) which binds with TRH receptors in pituitary cell membrane
- binding activates a phospholipase second messenger system inside the pituitary cells to produce large amounts of phospholipase C which results in a cascade leading to TSH release
How is the excessive secretion of T3 and T4 in response to cold temperatures inhibited and regulated?
negative feedback loop to the anterior pituitary as well as the hypothalamus from T3 and T4 to prevent TSH and TRH release and in turn, T3 and T4 release (to maintain a stable metabolism)
