Thyroid Gland Flashcards
Describe location and structure of thryoid gland
in the neck, in front of the lower larynx and upper trachea.
-Butterfly shaped w, 2 lateral lobes joined by a central ISTHMUS.
-2-3 cm and weighs 15-20g
-
Outline the overall structure of T3 & T4 and the
mechanisms of their production, storage and
secretion.
O
Describe how the activity of the thyroid gland is
controlled.
they r under the control of the hypothalamus & Anterior pituatary gland. TRH is released from the dorsomedial nucleus of the hypothalamus. under the influence of the:
- circulating T3 & T4. (negative feedback).
- Stress
- Temp (low temp enhances release of TRH)
TRH travels in the ‘hypothalamic-pituatry portal system to stimulate TSH release from thyrotropes in the Anterior pituitary.
TSH then travels in the blood to affect the FOLLICULAR cells of thyroid gland.
Describe the effects TSH when bound to the receptors on the follicular cells on thyroid gland
TSH: interacts with receptors on the follicle cells and will
1) increase synthesis of T3 & T4
2) increases follicle cell size
3) increases follicle cell proliferation
4) increase vascularity of follicle cells
THESE EFFECTS MAY LEAD TO GOITRE!
Which form is absorbed by the body?
Iodine or iodide?
Iodine is reduces into IODIDE befire absorbtion in S. Intestine
Where is most iodine contained in the human body?
90-95% thyroid gland
What is Goitre?
Enlargement of thyorid gland
Develops when thyroid gland is overstimulated!
How do u treat hyperthyroidism?
Antithyroid drugs
what r the 2 major cell types found in the gland?
- Follicular cells -
- parafollicular cells (C cells)>r found in the connective tissue
what r c cells?
parafollicular cells, they secrete Calcitonin hence the “C”
what are the only molecules in the human body that contain iodine?
Thyroid hormones
what is colloid? function?
Colloid stores thyroglobulin!!
glycogenous material present in the lumen of the follicular cells. within the colloid, it is having a lot of thyroglobins that has DIt and MIT residues
-made by FOLLICULAR CELLS
what is the membrane bound enzyme found in the apical side of follicular cells? what is it 3 functions.
THYROID PEROXIDASE “OAC”
Membrane bound enzyme that regulates 3 separate reactions involving iodide
1) Oxidation of Iodide to Iodine (requires the presence of H2O2)
2) Addition of Iodine to tyrosine acceptor residues on the protein throglobulin
3) Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin
function of thyroglobulin? where is it made?
Thryoglobulin acts as a scaffold on which thyroid hormones are formed
Act as a STORE of the tyrosine residues
made in follicular cells
which thyroid hormone is released more?
T4»90%
how does the amount of Iodide in ur blood contribute to the different formation of thyroid hormones?
the more the Iodide in ur blood, the more that enters follicular cells, the more DIT is made
DIT-DIT= T4
more T4 will be made rather than T3
bs alaa.
how is Iodide (I-) is taken up from blood? where does it go
it is taken into thyroid epithelial cells AGAINST its concentration gradient (follicular cells), which have a sodium- iodide symporter or “iodine trap”
iodide or iodine is absorbed in the GI? (pick one)
Iodide (I-)
where do u get most of ur iodine from?
the diet
Dairy products
Grains
Meat
Vegetables
Eggs
Iodized salt
Daily intake (μg)
52
78
31
20
10
380
where is most T4 is converted to T3
liver and kidneys
where does f 80% of circulating T3 come from?
T4
how r T3 &; T4 are transported in blood? (3)
and how do they exert their effects on cells?
theyre lipid soluble (hydrophobic), so they need to be bound to a protein transporter
- thyroxine-binding globulin
- Pre-albumin
- Albumin
And only 1% is present free in circulation.(it is the free form that is biologically active)
they cross the cells via diffusion or specific carrier»then bind to cytoplasmic binding proteins that take them to receptors located in the NUCLEUS
Most T4 is converted to T3 outside thyroid
ok
which hormones r produced in the thyroid gland?
T3, T4> follicular cells
Calcitonin> parafollicular cells (C-cells)
describe composition of T3 & T4
they r small molecules derived from the AA tyrosine with addition of atoms IODINE.
what is calcitonin?
Polypeptide hormone involved in Calcium metabolism.
How is T3 and T4 stored?
extracellularly in the lumen of the follicles as part of the thyroglobulin molecules. they would last for several months at normal rates.
Most commonist cause of goitre in UK?
Multinodular goitre.
Common causes of goitre?
1) IODINE DEFICIENCY> iodine i needed to make T3 & T4, Reduced thyroxine levels lead to increased TSH which leads to generlised thyroid enlargement.
2) Multinodular Goitre> Normal thryoid function, but a tiny % of ppl may go on in developing hyperthyroidism “toxic multi nodular goitre”
what is goitre?
when can u get physiological Goitre (normal thyroid function)?
if we say a patient has goitre it does not tell us anything about his FUNCTIONAL status
it just means that size of the gland is LARGE.
- Menarche (1st period)
- Preganancy
- Menopause
if large amounts of iodide is present in ur blood, the iodide transporters will become slow. and vice versa
BUT if u give soon much iodide to patient, the iodide pump can eventually FAIL to work
very smart in regulating the amount of iodide that enters our follicular cells
