Thyroid Flashcards
(21 cards)
Where is the thyroid located?
▪️Below the larynx and anterior to the upper part of the trachea.
▪️Isthmus overlies 2-4th tracheal cartilage.
What is the structure of the thyroid lobes?
▪️Contain follicles which are functional units and are surrounded by epithelial cells - filled with colloid.
▪️C-cells attached to follicles-secrete calcitonin (regulates calcium levels)
▪️Capillaries
▪️Surrounded by a capsule
Colloid
▪️ Made mostly of Thyroglobulin (glycoprotein made of Tyrosine)
Tyrosine and Iodine
▪️Tyrosine is an Amino Acid made in the Thyroglobulin of the Follicle cells.
▪️Iodine is a dietary essential
Biosynthesis, storage and secretion of thyroid hormones.
1️⃣Thyroglobulin is synthesised by epithelial cells of the follicles and is released into colloid by exocytosis.
2️⃣A) Iodide (-ive charge) is actively taken up by follicular cells from the blood.
2️⃣B) Iodide is the oxidised (no charge) into Iodine and transferred into colloid lumen.
3️⃣ Iodine is them attached to Tyrosine in colloid forming di- and mono-iodotyrosine (DIT and MIT)
4️⃣ Coupling between the iodinated tyrosine molecules form T4 (2DIT) and T3 (DIT+ MIT)
5️⃣T3 and T4 are then endocytosed and fused with a lysosome after stimulation.
6️⃣The lysosome separates T3 and T4 from normal thyroglobulin and the hormones diffuse from the follicle to the bloodstream and peripheral tissues.
Thyroid hormone
▪️90% of hormones released are T4
▪️In thyroid and peripheral tissues( liver, kidney) T4 is converted to T3 by monodeiodination. ( type 1 5’- deiodinase)
▪️T3 is much more potent in biologic form than T4, therefore interacts more with target cells.
Negative feedback of Thyroid hormones
1️⃣Hypothalamus (Thyroid releasing hormone TRH)
2️⃣Anterior Pituitary (Thyroid stimulating hormone TSH) NF to 1.
3️⃣Thyroid Gland(Thyroid hormone) NF to 1&2.
4️⃣Target cells
Mechanism of Thyroid hormone release
▪️TSH binds to G-protein coupled receptor on the thyroid follicular cell.
▪️Leads to adenylate cyclise converting ATP into cAMP.
▪️cAMP acts as a second messenger which triggers release of hormone into blood and tissues.
Thyroid hormone affect on growth
▪️ Can stimulate Growth Hormone (GH) secretion and promote GH effects
▪️Lack leads to Growth and Mental retardation can be reversed by thyroid replacement therapy days or weeks after birth.
▪️An excess does not produce excess growth
▪️promoted growth and development of brain during Fetal and postnatal life
Physiological effects of Thyroid hormones
▪️⬆️ Metabolic activities
▪️⬆️ Oxygen consumption to active tissues
▪️⬆️ Basal metabolic rate (BMR)- affects rate a person burns calories
▪️⬆️ Heat production
Sympathomimetic effect of Thyroid hormone
▪️ Any action similar to one produced by sympathetic nervous system
▪️⬆️ target cell sensitivity to Catecholamines
▪️⬆️ production of Catecholamine target cell receptors.
Thyroid hormones affect on cardiovascular system
▪️⬆️ Heart sensitivity to Catecholamines.
▪️⬆️ Heart rate and force of contraction -⬆️CO.
▪️⬆️ peripheral vasodilation to eliminate extra heat.
Symptoms of Hypothyroidism
▪️Mental slowness ▪️Lethargy
▪️Constipation ▪️Cold intolerance
▪️Mild weight gain ▪️Fatigue
▪️Dry skin
Signs of Hypothyroidism
▪️Goitre (Primary Hypothyroidism) ▪️Bradycardia ▪️Hypertension ▪️Slow movements- delayed reactions ▪️Voice hoarseness
Causes of Hypothyroidism
▪️ Autoimmune (Hashimoto’s) Thyroiditis
▪️Atrophic (common in elderly)- withers away
▪️Iodine 131 treatment- treats Hyperthyroidism (one extreme to the other)
▪️ Secondary Hypothyroidism- Pituitary
Test results from Hypothyroidism
▪️⬆️TSH (primary Hypothyroidism)
▪️⬇️ Low levels of T3 and T4
▪️⬆️ Antibodies targeted at thyroid ( anti-thyroid Peroxidase, anti-thyroglobulin)
Hypothyroidism therapy
▪️L-Thyroxine (manufactured form of thyroid hormone)- 100/125g
Mechanism of thyroid hormone and receptors
▪️ Nuclear receptors in cytosol
▪️ T3 and T4 can get through the lipid plasma membrane as they are small and lipid soluble
▪️ Need to enter with a cofactor (RA)
▪️Binds to receptors inside the cells
▪️Receptor then relocate into the nucleus and bind to DNA
▪️Promote gene expression
Causes, symptoms, signs of Hyperthyroidism ( Thyrotoxicosis)
▪️Excess production of Thyroxine
▪️Autoimmune
▪️Tachycardia- Palpitations
▪️Weight loss
▪️Heat intolerance
▪️Sensitises body to adrenaline-constant adrenaline rush
▪️Goitre
Thyroid gland development
▪️Week 3, endodermal thickening on the midline at the back of tongue between turberculum impar (A) and copula (P).
▪️Later thickening becomes a diverticulum (outgrowth) that grows downtime neck. Thyroglossal duct is formed.
▪️Distal end dilates and grows and becomes thyroid gland. Duct becomes solid cord of cells.
▪️Week 7,
thyroid reaches position in the neck. Solid cord connecting to neck disappears. Foramen cecum (pit) remains on tongue.
▪️Paramidal lobe can remain on thyroid gland- where it was connected to tongue.
Thyroglossal Cysts
▪️Thyroglossal duct does not close during development.
▪️Mucus secreted can enter the duct which grows as a cysts.
▪️ Cysts can get infected.
▪️ Removed surgically.
