7 The Thyroid And Iodothyronines Flashcards
Q: What is the thyroid gland anatomy? Shape?What is included in its structure? (2) Full on?
A: at top of trachea in neck
Bi-lobed
ISTHMUS = the bit of tissue that joins together the two lobes
PYARMID = the bit of tissue on the top of the isthmus which is present in some people
Thyroid is full of follicles
Q: Describe the structure of thyroid follicles. (3)
A: Follicular cells = the cells lining the follicle
Colloid = proteinaceous substance found inside the follicles
Parafollicular Cells = found between the follicles and they also provide hormones
Q: What produces iodothyronines?
A: follicular cells
Q: How are iodothyronines made? Include diagram.
A: anterior pit produces TSH and TSH binds to TSHR -> lots of effects (Thyrotrophin Receptors are found on the serosal/basal membrane of follicular cells)
1 sodium-iodide (negative 1 iodine ion) pump= stimulated -> iodide pumped into cells and then out the cell via pendrin pumps (IODIDE HAS BEEN MOVED FROM BLOOD TO THE COLLOID)
2 nucleus stimulated -> thyroglobulin (TG) produced -> moved to colloid and remains associated with the apical membrane
3 thyroid peroxidase (TPO) enzyme is stimulated and in the presence of hydrogen peroxide it converts iodide into a reactive iodine form which is very short lived (at apical membrane)
4 TSH keeps stimulating TPO which allows COUPLING REACTIONS to take place where Triiodothyronines (T3) and Tetraiodothyronines (T4) can be formed
The thyroid hormones are produced on the apical membrane
5 TSH then stimulates lysosomes to move towards the apical membrane and stimulates the uptake of colloid by the apical membrane (colloid is a massive store of iodothyronines)
6 The colloid that has been internalised then fuses with the lysosome and the enzymes break down the protein, liberating T3 and T4
7 T3 and T4 move out in to the blood
Q: What does having an overactive thyroid mean? Treatment?
A: secreting too much thyroxine
given carbimazole
Q: How do you get T3 and T4?
A: MONOIODOTYROSINE (MIT) is where you add 1 iodine and DIIODOTYROSINE (DIT) is where you add 2 iodines
then get coupling reactions
DIT + DIT = T4 aka thyroxine
DIT + MIT = T3
Q: How are T3 and T4 transported around the body?
A: mainly bound to plasma proteins
a) thyroid-binding globulin: TBG (70-80%) = specific to T3/4
b) albumin (10-15%)
c) prealbumin (aka transthyretin)
very little is unbound
Q: What percentage of T3 and T4 are unbound? What does this mean?
A: T3- 0.5%
T4- 0.05%
bioactive form
Q: What’s the main hormone product of the thyroid gland? What happens to it in target tissue?
A: T4
deiodinated to T3 (bioactive form) in target tissue
(some T3 is made but most is made as a result of T4 deiodination)
can be deiodinated in different position to get reverse T3 (inactive)
Q: Draw simple diagrams for thyrosine, T3, reverse T3 and T4.
A: DIAGRAM
Q: What is the mechanism for thyroid hormone action?
A: Iodothyronines taken up by cells via transporter
any t4 that enters is converted to t3
some t3 that enters causes non-nuclear actions on ion channels
t3 main effect is binding to THR in the nucleus where receptor-hormone complex affects transcription of various genes and protein synthesis eg hormones/receptors
t3 stimulates metabolic activity so has a lot of effects on the mitochondria
IMPORTANT: Iodothyronines have a GENOMIC ACTION
Q: What is important to remember about iodothyronine action?
A: action is genomic
Q: What are the actions of TH?
A: fetal growth and development
untreated congenital hypothyroidism: cretinism
TH and TSH measured in new born infant’s heel-prick test
increase basal metabolic rate (-> lose weight)
protein, carbohydrate and fat metabolism
potentiate actions of catecholamines (eg tachycardia, lipolysis)
effects on the GI eg diarrhea, CNS, reproductive system
Q: What is the latent period for T3 and T4?
A: t3- around 12hrs
t4- around 72hrs
Q: What is the half life of T3 and T4?
A: t4- 7-9 days
t3- 3 days
