The Thyroid and Iodothyronines Flashcards
Where is the Thyroid located?
Top of Trachea
How many lobes has the Thyroid?
2
What is the tissue that joins the two lobes of the thymus together called?
The Isthmus
What is the thyroid pyramid?
The bit of tissue on the top of the isthmus which is present in some people (important clinically because a thyroid tumour could be in the pyramid)
What are the three main components of the thyroid?
Follicular cells
Colloid: Proteinaceous substance found in the inside of follicles
Parafollicular cells: Found between the follicles and they also provide hormones
How does iodine enter Follicular cells?
Iodine pumps on basolateral surface from blood
What happens to iodine that enters follicular cells?
It is pumped into the colloid by Pendrin pumps, to be used in hormone production
(This is stimulated by TSH)
What are the four effects of TSH on a follicular cell?
- Stimulation of iodine pumps
- Stimulation of nucleus to produce thyroglobulin
- Stimulation of Thyroid Peroxidase (TPO) enzyme
- Stimulates movement of lysozymes towards apical membrane to collect colloid to secrete into blood
What does Thyroid peroxidase do?
(TPO)
TPO converts iodine into a short-lived reactive form using H2O2
also
TPO allows coupling reactions where T3 & T4 are formed
What is thyroglobulin?
Where does it go once synthesised?
Long array of amino acids that have a certain number of Tyrosyl residues which become iodinated by reactive iodine.
Remains associated with the apical membrane
What two products can result from the iodination of Tyrosyl residues?
MONOIODOTYROSINE (MIT)
and
DIIODOTYROSINE (DIT)
What are the products of coupling reactions by TPO?
Triiodothyronines (T3)
and
Tetraiodothyronines (T4)
What is Tetraiodothyronine also known as?
Thyroxine
How do iodothyronines get into the blood?
Movement of T3/T4 bound to TG into lysozymes at the apical membrane
Enzymes remove TG, liberating the hormones which can then move out into the blood.
How are Iodothyronines transported in the blood?
Bound to plasma proteins to prevent uptake by non-target cells.
Thyroxine-Binding Globulin is specific to T3/T4, but can also weakly bind to albumin
What are the latent periods and half-lives of Thyroid hormones?
Latent Periods:
T3 = 12 hours
T4 = 72 hours
Biological Half Lives:
T3 = 2 days
T4 = 7-9 days
What is the main hormone product of the thyroid, and what is the most active thyroid hormone?
What does this mean?
Main hormone product: T4
More bioactive hormone: T3
Therefore T4 is deiodinated in target tissues
What is reverse T3?
T4 which has been deiodinated in a different position formin rT3 which is biologically inactive
What is the main action of Iodothyronines?
Increases Basal Metabolic Rate (BMR)
(Except in the brain. Useful for calorigenesis which is used in temp reg)
Increase Protein, Carbohydrate, and Fat metabolism
Potentiates action of catecholamines leading to tachycardia and lipolysis - may present with palpitations
Some effects on GI/CNS/Reproductive
What is the effect of congenital hypothyroidism?
If untreated within a few months, leads to cretinism.
Test for TH/TSH in heel prick test
Describe the control of TH production:
(hypothalamo-pituitary-thyroidal axis)
Thyrotrophin Releasing Hormone (Hypothalamus)
→
Thyrotrophin (TSH) (adenohypophysis)
→
Iodothyronines (T3 & T4) (Thyroid gland)
Describe the negative feedback of Iodothyronine production
T3/T4 directly inhibits Anterior pituitary Thyrotrophin production and indirectly inhibits Thyroid releasing hormone from hypothalamus
