6 - The thyroid and iodothyronines Flashcards
How many lobes does the thyroid gland have?
2
Where is the thyroid gland located?
On the trachea at the front of the neck.
What are the 2 physical structures in the middle of the thyroid gland called?
The pyramid
Isthmus
Where are the parathyroid glands found? What do they release?
At the top and bottom of each lobe.
They release PTH (parathyroid hormone)
How are the cells in the thyroid arranged?
Follicles - Follicular cells arranged around the colloid.
Parafollicular cells are arranged around the follicles
What do parafollicular cells secrete?
Calcitonin
What receptor does TSH bind to?
TSHR
TSH receptor
What does this TSH stimulate?
- Iodine uptake into the follicular cell through the basolateral membrane.
- Synthesis of thyroglobulin (TG)
What happens to the iodine that inters the follicular cell?
It leaves through the apical membrane into the colloid.
It is added to amino acids (tyrosine).
What is iodination?
When iodine is added to amino acids (tyrosine)
What are MID and DIT?
MID - monoiodotyrosine
DIT - diiodotyrosine
What is a coupling reaction?
When MID and DIT bind:
MID + DIT = T3 (tri-iodothyrine)
DIT + DIT = T4 (tetra-iodothyrine)
What happens to T3 and T4?
They are secreted into the blood through the basolateral membrane?
What must happen to T4?
It must be converted into an active form of T3 (deiodinated).
Why is there an active and inactive form of T3?
The iodines are in the incorrect position in the inactive T3.
What is the mechanism of action of T3?
- Enters cell
- Stimulates metabolic activity in mitochondria
- THR receptor
- Transcription & protein synthesis
How are thyroid hormones transported in the blood?
Bound to plasma proteins
- Thyroid binding globulin (TBG)
- Albumin
- Prealbumin
Some are unbound (bioactive components)
What are the actions of TH?
- Fetal growth & development
- Untreated congenital hypothyroidism: cretinism
- TH and TSH measured in new borns heel prick test.
- Increase basal metabolic rate
- Protein carb and fat metabolism
- Potentiate actions of catecholamines (tachycardia, lipolysis)
- Effects on GI, CNS, reproductive system.
What are the latent periods for T3 an T4?
T3: 12 hours
T4: 72 hours
What are the half lives of T3 and T4?
T3: 2 days
T4: 7-9 days
Draw the control of TH production axis.
Drawing
