Thyroid Gland Flashcards
(25 cards)
Name the structures within the thyroid gland
Colloids surrounded by follicular cells
Follicles
Parafollicular cells
Which gland embedded in thyroid
Parathyroid glands
Which nerve runs close to the thyroid
Left recurrent laryngeal nerve (supples vocal chords)
Explain the structure of the thyroid gland, with reference to its embryo logical origins
Midline out pouch from floor or pharynx (originates from base of tongue) Development of thyroglossal duct Divides into 2 lobes Duct disappears leaving for amen caecum Final position by week 7 Thyroid gland then develops
Summarise the process of TSH release to thyroid hormone synthesis
Iodine from blood transported into the thyroid by sodium iodine pumps in the follicular cells.
TSH attaches to TSH-R on the membrane causing the release of thyroglobulin TG from the nucleus and the release of the enzyme thyroid peroxidase TPO
Iodine then transported into the colloid via transport protein, undergoes iodination, then again with TG forming TG MIT + DIT. TG MIT DIT undergoes a coupling reaction to form T3 and T4 thyroid hormones .
All the reactions in the colloid (iodination and coupling reactions) are catalysed by TPO and mediated by hydrogen peroxide
Thyroid hormones are then transported back to the blood via the follicular cell .
Full names of structure TG MIT DIT
Tyrosine attached to 3-monoiodotyrosine and 3,5-diiodotyrosine
How is T3 formed using names of iodothyronines
3-monoiodotyrosine attaches to 3,5-diiodotyrosine to form 3,5,3’-tri-iodothyronine AKA T3
MIT+DIT= T3
How is T4 formed using names of iodothyronines
3,5-diiodotyrosine attaches to another 3,5-diiodotyrosine to form 3,5,3’,5’-tetra-idothyronine AKA thyroxine AKA T4
DIT+DIT=T4
When does deiodination take place?
Thyroxine T4 is the main hormone produced in the thyroid gland
T4 is a prohormone deiodinated by deiodinase enzyme into triiodothyronine T3, the bioactive form providing almost all the thyroid hormone activity in target tissues
(Also possible to deiodinate T4 to form reverse T3 the inactive version - depending on which iodine is removed)
Ratio of T3 from thyroid gland directly vs deiodination of T4
20% made in thyroid directly vs 80% made by T4 to T3 conversion in peripheral tissue by deiodinase enzymes
How are thyroid hormones transported in the blood
Mostly bound to plasma proteins: thyroid-binding globulin TBG 70-80% albumin 10-15% prealbumin (transthyretin) Only 0.05% T4 and 0.5% T3 is unbound (bioactive components)
How do thyroid hormones affect gene expression?
Any T4 converted to T3 of which attach to thyroid hormone receptor enzymes in the nucleus (TR on TRE) which alters gene expression
Purpose of thyroid hormones, lack of causes what and how to test levels
Essential for feral growth and development, particularly the CNS
untreated congenital hypothyroidism = cretinism
TSH measured in new borns via heel-prick test
Actions of thyroid hormones
Increase basal metabolic rate
Protein, carb and fat metabolism
Potentiate actions of catecholamines (tachycardia, lipolysis)
Effects on GI, CNS, reproductive systems
Half lives of T3 and T4
T3 - 2days
T4 - 7-9days
How is thyroid hormone production controlled?
Using ideas of negative feedback when detecting certain levels of the
Controlling (increase/decrease) amount of Somatostatin (SS) or TRH released at hypothalamus, amount of TSH released at anterior pituitary or amount of iodide at the Thyroid which all in turn affects the amount of T3 and T4 produced
Wolff-Chaikoff effect
The reduction in thyroid hormone synthesis caused by ingestion of a large amount of iodine in the follicular cells
Hypothyroidism
Autoimmune damage to the thyroid causing thyroxine levels to decline and TSH levels to climb
Most common thyroid disease - Hashimoto’s thyroiditis
Symptoms of hypothyroidism?
Deepening voice Depression and tiredness Hair loss Enlarged thyroid gland Swollen face Weight gain with reduced appetite Constipation Dry rough skin Bradycardia Muscle cramps
Levothyroxine:
Mechanism/ Indication/ Dosing/ Potential Complications
Provides T4 instead of thyroid which can then be deiodinated into T3
Can be used for both hypo (increase T3 production) and hyperthyroidism (blocking and replacing)
Dosing adjusted according to TSH, common dose 100mg and commonly administered orally
Minor complications = weight loss and headache
Major complications = tachycardia and heart attack (all side effects rare)
Patients adamant to have Liothyronine (T3 treatment), why not give that straight away?
No difference in action of T4 and T3
T4 converted to T3 in the body anyways
T3 drugs however are much more expensive
Why not try combined thyroid hormone replacement?
T4 = prohormone, converted by deiodinase action to T3
Combination T4/T3 has some reported improvement in wellbeing
However complicated by symptoms of ‘toxicity’ - palpitations, tremors, anxiety and often suppresses TSH
Hyperthyroidism
Thyroid makes too much thyroxine causing levels to rise and TSH levels to drop
Most common autoimmune disease: Graves’ disease where whole gland smoothly enlarged and overactive
Grave’s Disease symptoms
Antibodies bind to and stimulate TSH receptor in the thyroid causing smooth goitre, diffuse enlargement of thyroid gland
Antibodies bind to muscles behind eye causing exophthalmos (being able to see iris all around eye)
Antibodies stimulate growth of soft tissue of shins aka pretibial myxoedema
