Thyroid Gland Flashcards
Where is the thyroid gland located?
- Located in the neck
- Below the thyroid cartialge
- Moves up when you swallow
What is the morphology of the thyroid gland?
- Butterfly shaped gland
- 2 lobes
- Central portion = Isthmus
What cells and other components is the thyroid gland made of?
- Structures called follicles (spheres)
- Follicular cells
- Middle portion is colloid (thick mucus like ECF)
- Parafollicular cells : release calcitonin for calcium metabolism
What structures are in close proximity to the thyroid gland?
- Parathyroid glands embedded in thyroid (important when doing surgery to remove thyroid gland to keep parathyroid gland - calcium metabolism)
- Left recurrent laryngeal nerve runs close (supplies vocal cords & controls voice)
What is the embryology of the thyroid?
- Midline outpouching from floor of pharynx (originates from base of tongue)
- Development of thyroglossal duct (runs all the way down neck)
- Divides into 2 lobes
- Duct disappears leaving foramen caecum
- Final position by week 7 ; thyroid gland deveops
What can go wrong in thryoid development?
- No development of gland (throid agenesis)
- Process stopping along any point in thryogloosal duct, doesnt reach final position
- Lump in throat, surgeon removes, could remove only source of thyroid production and render them hypothyroid
- Lingual thyroid
What is the histology of the thyroid?
- Round structures are follicles
- Purple dots are nuclei of each follicular cell
- Red blobs are capillaries - blood supply of thryoud
- Pink - Colloid
- White patch on follicluar cells - site of thyroid hormone production
What does the binding TSH to TSH receptor lead to?
- Activates sodium iodide transporter which allows iodide ions to enter follicular cell and then enter colloid, iodide oxidised in iodination
- Activates production of pro-hormone thyroid globulin which enters colloid
- Activates enzyme called thyroperoxidase TPO. TPO enters colloid and llong with hydrogen peroxide catalyses iodination reactions
- Catalyses reaction called coupling reaction
How does the coupling reaction that produces T3 & T4 occur?
- Thyroglobulin is pro-hormone made of amino acids including tyrosine (aromatic amino acid)
- Add iodine to tyrosine = 3-Monoiodotyrosine
- 2 Iodine to tyrosine = 3,5-Diiodotyrosine
- 3-Mono + 3,5-Diiodo = 3,5,3’-Tri-iodothyronine (T3)
- 2 3,5-Diiodo = 3,5,3’,5’-Tetra-Iodothyroxine (T4, Thyroxine)
What is the deiodination process that leads to T3?
- Prohormone thyroglobulin (series of amino acids) with tyrosine residue (100 maybe but 20 can be iodinated)
- Deiodinate T4 gives T3
- Position of iodine molecule important, wrong person gives inactive form (reverse T3)
What is the proportion of thyroid hormones T3 & T4?
- Healthy adult thyroid gland secretes T3 & T4
- T4 prohormone converted by DEIODINASE ENZYME into more active metabolite TRIIODOTHYRONINE (T3)
- Circulating T3: 80% from deiodination of T4 , 20% from direct thyroidal secretion
- T3 provides amost all the thyroid hormone activity in target cells
How are thyroid hormones T3 & T4 transported in blood?
- Mostly bound to plasma proteins:
a) Thyroid-binding globulin : TBG (70%-80%)
b) Albumin (10%-15%)
c) Prealbumin (aka transthyretin) - Only 0.05% T4 and 0.5% T3 is unbound (bioactive components)
How do thyroid hormones have an effect on gene expression?
- T3 & T4 get to target cells and enter cell via relevant receptor
- T4 deiodinated by deiodinase enzyme (present in all target cells) to T3
- T3 enters nucleus via nuclear receptor & has effect on gene expression
- Target cell carries out task
What affect does thyroid hormone have on foetuses and what happens when they don’t have enough?
- Essential for fetal growth & development, in particular CNS
- Untreated congenital hypothyroidism : cretinism
- TSH measured in new-born infants heel prick test
(normally get thyroid hormone from mother)
What does thyroid hormone do?
- Increases basal metabolic rate
- Protein, carbohydrate & fat metabolism
- Potentiate actions of catecholamines (e,g, tachycardia, lipolysis)
- Effects on the GI, CNS, Reproductive systems
- Thyroid hormones binds nuclear receptor -> synthesis of new proteins