The Thyroid gland and Iodothyronines Flashcards
How many lobes in the thyroid gland?
2
Left and Right
What does thyroid tissue consist of?
Follicles which consist of a central colloid, surrounded by follicular cells surrounded by parafollicular cells
What is between the 2 lobes of the thyroid?
Isthmus
Where is the thyroid?
Infront of the trachea
What is the thyroid shaped like?
A butterfly
TSH secreted from anterior pituitary stimulates Thyroid gland
TSH binds to TSH receptor on follicular cells, stimulating various processes
What processes are stimulated by the binding of TSH to thyroid follicular cells?
Uptake of iodide
Synthesis of Thyroglobulin
Iodination of thyroglobulin
Coupling reaction
How and why is iodide taken in by the thyroid?
Need iodide for thyroid hormones
Taken up from blood through sodium-iodide symporters
Iodide transported into colloid
What is thyroglobulin?
Protein made of AAs
What is iodination of Thyroglobulin?
Add iodide to Tyrosine residues in Thyroglobulin
What catalyses iodination of Thyroglobulin?
Thyroperoxidase (TPO)
What can be produced from iodination of Thyroglobulin?
+1 Iodide = Mono-iodo-tyrosine (MIT)
+2 Iodides = Di-iodo-tyrosine (DIT)
What is produced in a coupling reaction of MIT + DIT?
T3
What is produced in a coupling reaction of DIT + DIT?
T4
After production in the colloid, where are T3 and T4 taken?
Back into follicular cells
Secreted into blood when required
What is the predominant hormone secreted by the Thyroid?
T4
Thyroxine
Iodothyronines
Thyroid hormones
How are iodothyronines mainly transported?
Bound to plasma proteins in blood
What are the proportions of binding of iodothyronines?
Thyroid binding globulin (TBG)= 70-80%
Albumin= 10-15%
Prealbumin= Small %
Percentages of unbound iodothyronines
- 05% T4
0. 5% T3
How is T3 different to T4?
T3 is the bioactive form of Thyroid hormone
What is formed from de-iodination of T4?
T3
Occurs in target tissues
A little T3 is made in Thyroid
Majority of T3 made in peripheral tissue from de-iodination of T4
T4 can be de-iodinated in a different position
Produces reverse T3
inactive form
Mechanism for main action of T3
T3 binds to Thyroid hormone receptor
Which stimulates transcription of genes, leading to protein synthesis (in nucleus)
T3 May also have non-nuclear actions
Bind to ion channels on cell surface membrane
Thyroid hormone effects
Fetal growth and development
Basal metabolic rate
Congenital hypothyroidism
=Cretinism
So TH & TSH measured in heel prick test
Hyperthyroidism
Increases basal metabolic rate (protein, carbs, fat)
Potentiates actions of catecholamines- Tachycardia
Effects on GI, CNS, Reproductive system
What effects does hyperthyroidism on GI, CNS and Reproductive system?
GI= Diarrhoea
CNS= Agitation
Reproductive system= Irregular periods
Latent periods of thyroid hormones
T3= 12 hours T4= 72 hours
Half-lives of thyroid hormones
T3= 2 days T4= 7-9 days
Hyperthyroidism TSH levels
Expect low levels TSH
Hypothyroidism TSH levels
Expect high levels TSH
What does TRH stimulate?
TSH
What does TSH stimulate?
Production of T3 and T4
What is the negative feedback loop in Thyroid hormone production?
T3 and T4 inhibit production of TRH and TSH
What is the Wolff-Chaikoff effect?
Iodide itself may inhibit Thyroid hormone release
What hormone may stimulate TRH production?
Oestrogen
What may suppress TRH production?
Glucocorticoids
