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
why is pottassium iodide used before surgery?
this will inhibit the iodide transporters> but will not form GOITRE it will rather reduce the size of thyroid gland and its function and vascularity» making surgical approach easier.
which hormone binds more tightly with the thyroxine-binding globulin, what does that mean for its half life?
T4 binds more tightly so it will remain in the circulation much longer!
1/2 life of T4»approx 8 days
………………T3»approx 2 days
Explain the different effects that pregnancy has on thyroid hormones.
OESTROGEN increases the synthesis of TBG during pregnancy & this produces a fall in the amount of Thyroid hormones in the circulation bc they r bound.
this will increase TSH levels and the hormones go back to normal, BUT the total amount in the blood will be increased.
how can cirhosiss of liver have an effect on T3 & T4
the thyroxine-binding globulin is made in LIVER. if liver is dysfunction , Less amounts of TBG is formed» less T3 & T4 can be carried and levels DROP.
what is the half life of TSH? when can stimulation of TSH receptors be increased?
1 hour,
in conditions like gRaves disease, where TSI r made and overstimulate the TSH receptors.
what r the short & long terms Of TSH acting on their receptors on follicular cells?
short term» there is increase
Long term» stimulation of follicular cells to overgrow and overstimulate undergoing HYPERTOPHY, the cell will change its shape from cuboidal to columnar, the lumen will be crowded and tight.
If T4 is Low
TSH will be»_space;»>?
high
how does stress and heat effect amounts of TSH in our body
there r neurons in the hypothalamic area that r “temp sensitive”, if ur body temp is less, those neurons will contribute to be releasing more TRH will will eventually lead to increased released of TSH from Anterior pituitary!
when u r stressed> sympathetic increases> ur body temp increases> heat is produced> this INHIBTS the release of TRH> reduced TSH
how is thyroglobin taken up from the lumen into the follicular cells? what happens to them there
by the process of endocytosi, the r cleave by lysosomal enzymes and release T3 & T4 which will diffuse in to the circulation
what is TSH? describe its structure? when do its levels peak
glycoprotein hormone consisting of 2 non-covalently linked subunits (A & beta)
Higher in NIGHT
Low in morning
how do thyroid hormones have an effect on physical growth?
they directly effect BONE MINERALISATION and increase synthesis of heart muscle protein.
In adults Lack of thyroid hormones is characterised by poor concentration, confusion, increased motility, Thick puffy skin what is this called?
Myexdema.
describe the INDIRECT actions of T3 &; T4
they have interaction with hormone and neurotransmitters where they stimulate their responsiveness and synthesis in some tissues»
HEART» Tachycardia
GI»increased motility
what is the most common form of Hypothyrodism?
hashimostos disease
autoimmune disease that results from destruction of thryroid follicules or production of antibody that block TSH recepters on follicle cells.
what r the actions of carbimazole?
it inhibits THROID PEROXIDASE!
what r the actions of carbimazole?
its converted to Methimazole in cell and inhibits THYROID PEROXIDASE!
how do thyroid hormones Increase in Basal metabolic rate and heat production? Stimulation of metabolic pathways?
- Increasing the number and size of mitochondria
- Stimulating the synthesis of enzymes in the respiratory chain
Catabolic pathways are generally stimulated more than anabolic.
- Lipid metabolism: Stimulates lipolysis and β-oxidation of fatty acids
- Carbohydrate metabolism: Stimulate insulin-dependent entry of glucose into cells and increase gluconeogenesis and glycogenolysis
Thyroid hormones Increases target cell response to catecholamines by ….?
increasing receptor number on target cells
how do image for the thyroid gland?
Technetium-99m (99mTc) used for isotope scanning of the thyroid with a gamma camera.
- most commonly used.
- half life of 1 day
- Radiation exposure therefore low
How lond does is take for Carbimazole to exert an effect?
Delayed effect because ewe got a big store of thyroid hormone in thyroglobulin. so it takes time for this store to be depleted
what would u expect the TSH levels to be in a patient with Graves disease?
Below the normal range
describe formation of thyroid hormones T3 & T4
2 tyrosines linked together with iodine at three or four positions on the aromatic rings.
MIT +DIT= T3 Triiodothyronine
DIT + DIT = T4 Tetraiodothyronine
what determines whether T3 or T4 is made?
depends on number of iodine present.]
the more»_space; more T4
when can u get these findings?
TSH low
T3 & T4 low
hypothalamic or anterior pituitary failure.
no Goitre present
how can we image the thyroid gland?
ultrasound
REMEMBER
Technichium is treated the same way as iodine
i have no idea what London meant in that statement
Is colloid extracellular or intracellular
Colloid is “extracellular” even though it is inside the follicle
what is medullary thyroid carcinoma?
a tumor of the parafoliicular cells (C-cells).
patients present w/ muscle tetany due to HYPOCALCEMIA from excess calcitonin secretion
define hyperthyrodism, hypothyroidism and thyrotoxicoisis
Hyper: refers to overactive THYROID GLAND
Hypo:refers to underactive thyroid gland
thyrotoxicoisis: refers to the CLINICAL SYMPTOMS due to high levels of thyroid hormone in the BS
Compare and contrast the symptoms of thyrotoxicosis with those of simple anxiety
Some symptoms are quite similar
– Palpitations
– Restlessness
– Increased bowel movements – Tremor
Some differences…
– No goitre/proptosis
– May not have increased appetite and weight loss
– Thyrotoxicosis you see vasodilation (warm and sweaty)
– Anxiety you see vasoconstriction (cold and clammy)
why can hyperthyrodism cause tachycardia and can result in hypertension?
Thyroid hormones UPREGULATE the number of adrenoceptors in the body
• For example…
– Increased beta-adrenergic receptors in the heart results in tachycardia and can result in hypertension
function of TSH
In effect, the TSH level tells us what the patient’s own brain ‘thinks’ of the patient’s thyroid function. If the TSH is raised, then the thyroid gland is underfunctioning (hypothyroidism). If the TSH is low (supressed), then the thyroid gland is overfunctioning (hyperthyroidism)
ok
what is autoimmune disease? which autoimmune diseases effect endocrine glands?
body produces antibodies and sometimes T cells against urself!
-Islets of Langerhans – Type 1 diabetes
• Thyroid – Hypothyroidism (Hashimoto’s disease)
– Hyperthyroidism (Grave’s disease)
- Adrenal glands – Addison’s disease
- Autoimmune pituitary disease
- Autoimmune ovarian and testicular disease
autoimmune endocrine diseases are more common in women. Why?
n
Prevalence of Goitre in the UK? btw men and women
7%>FEMALE
1% male
It what type of ppl is iodine deficiency a strong concern?
in pregnant women!
If a mother is iodine deficient and hypothyroid then the foetus is also iodine deficient.
This leads to a child with: – Mental retardation – Abnormal gait – Deaf-mutism – Short stature – Goitre – Hypothyroidism
what happens when TSH is released in the blood stream?
it goes and bind on Gs rececepter on follicular cells» AC > CAMP> PKA>
OR
it goes and bind on Gq rececepter on follicular cells» PLC> PIP> DAG and IP3>
these stimulate the uptake of T3 and T4 into follicular cells via endocytosis
what is half life of TSH?
1 hr
in circulation T___ is more than T____ this means that feedback levels r dependent on T_____levels
T4 is more than T3»>
T4 levels
how do thyroid hormones enter cells?
pic
Thyroid hormones r
1) lipid soluble
B) water soluble
lipid soluble
the receptors in the nucleus or mitchondria of cell r______and have a more affinity to T3 Or T4?
proteins
T3
__________________is characterized by a low or undetectable concentration of serum TSH with free triiodothyronine and free thyroxine levels within laboratory reference ranges
Subclinical hyperthyroidism
Which of the drug is commonly associated with side effects that disrupt thyroid function? Why
Amiodorone
Bc it has a similar structure to thyroxine & can disrupt thyroid function
What is the commonest casue of goitre on a global basis
Iodine deficiency
Ovulation fails to occur in the absence of thyroid hormones.
Ok
Conversion of T4 to T3.
T4 can be converted to T3 in tissues by removal of the 5 ‘-iodide.
Conversion of T4 to T3? How
T4 can be converted to T3 in tissues by removal of the 5 ‘-iodide.
