The Thyroid Gland: Control of Secretion, Effects of the Thyroid Hormones Flashcards
Where does the thyroid arise from?
Thyroid tissue arises in the midline at a point on the tongue later known as the foramen caecum
Epithelial cells sink downwards anterior to the hyoid and larynx
What connects the thyroid to the tongue?
Thyroglossal duct
What cells can be seen in a thyroid gland histology?
Follicles
Follicular cells
Colloid
Parafollicular C-cells
Function of follicles in thyroid?
Responsible for the production and secretion of the thyroid hormones thyroxine (T4) and triiodothyronine (T3)
Function of colloid in thyroid?
Through the action of thyroid peroxidase, thyroid hormones accumulate in colloid, on the surface of thyroid epithelial cells
Function of parafollicular C-cells?
Parafollicular cells, also called C cells, are neuroendocrine cells in the thyroid. The primary function of these cells is to secrete calcitonin
What pathology can develop in the thyroglossal cyst?
Should close during development
If does not fully close develop into thyroglossal cyst
Describe the hypothalamo-pituitary control of thyroid homrone release?
Small-bodied neurons in the arcuate nucleus and median eminence synthesize and secrete thyrotropin releasing hormone
Long portal vessels carry TRH to the anterior pituitary
TRH binds to receptors on thyrotroph causing G-protein coupled receptors which stimulates IP3 which releases calcium stores
Thes then cause a release of TSH from the thyrotroph
This TSh then travels to the follicular cell cytosol and causes release of thyroxine
What inhibits the release of TSH?
Somatostatin
Dopamine
What are the different thyroid hormones?
Thyroxine (T4)
Triiodothyronine (T3)
Reverse T3
Peptide backbone of thyroglobulin molecule
What happens to the thyroglobulin molecule in the follicular cell?
Inside lysosomes of the follicular cells, enzymes will cleave the two peptide bonds shown releasing T4
Where does T3 and T4 feedback to?
Anterior pituitary
Arcuate nucleus
What are the 8 steps in synthesis and secretion of T3 and T4?
TSh increases thea citvity of NA/I cotransporter on the basolateral membrane of the thyroid follicular cell. The result is increased iodine trapping: the ratio of follicular-cell iodine to plasma iodine increases under conditions of high TSH
Iodide leaves the cell, probably via pendrin and enters the lumen. The follicular cell also secretes thyroglobulin. Thyroid peroxidase, on the luminal surface of secretory vesicles, oxidizes I^- to I^0
Tsh also stimulates iodination of thyroglobulin in the follicular lumen
TSh stimulates the conjugation of iodinated tyrosines to form T4 and T3 linked to thyroglobulin
TSh stimulates the endocytosis of iodinated thyroglobulin into the follicular cells from thyroid colloid
TSH stimulates the proteolysis of the iodinated thyroglobulin, forming T4 and T3 in the lumen of the lysoendomesome
FInally TSH exerts a growth factor effect, stimulating hyperplasia within the thyroid gland
What is the function of 5’/3’ and 5/3 monodeiodinase?
Cleaving iodine from T4 and T3
This can activate and deactivate thyroxine in the blood
What are the different mechanisms of action of thyroid hormones?
Increased sodium/potassium pump
Gluconeogenic enzymes
Respiratory enzymes
Myosin heavy chain
Beta adrenergic receptors
Mainy others
How does T3 and T4 enter cells and alter gene make up?
Both pass from blood via diffusion or carrier-mediated transport
Monodeiodinase removes the 5’ iodine converting T4 to T3
Thes then enter nucleus and bind to thyroid hormone receptor
This then alters the thyroid response element causing a change in transcription and therefore a change in production of mRNA
What is T3 and T4 bound to when carried in the blood?
TBG (thyroxin-binding protein)
What are the physiological actions of the thyroid hormones
Increased basal metabolic rate
This increases oxygen consumption
Which increases heat production
How does hyperthyroidism affect BMR?
Increases up to 100%
How does hypothyroidism affect BMR?
Decreases to 50-60%
What is the direct and indirect actions of T3 and T4 by mobilisation of carbohydrate, fat and protein stores?
Increased glucose uptake from GI tract
Increased glucose utilisation
Increased liver glycogenolysis and gluconeogenesis
Increased lipolysis in adipose tissue
Increased tissue oxidation of FFA
General increase in protein turnover
Increase in specific enzymes/ membrane proteins, hormone receptors
Permissive actions for other hormones (GH, prolactin, gonadal, adrenal steroids)
Essential for normal development and function of central and peripheral nervous system
What is T3 better at?
Genetic editing
This is why T4 is cleaved to from T3 prior before dissolving into the nucleus
Symptoms of hypothyroidism?
Poor mental ability
Lack of memory and initiative
Symptoms of fetal hypothyroidism?
Neuronal hypoplasia
Delayed myelination
Mental retardation
