Thyroid gland Flashcards
Vertebral level of thyroid gland
C5-T1
What does the thyroid gland produce (3)
Thyroid hormones, of 2 forms
- T3 (triiodothyronine)
- T4 (thyroxine)
Calcitonin
The hormones T3 (triiodothyronine) and T4 (thyroxine) are produced by the thyroid gland - what do the numbers refer to
the number of iodide ions attached to the hormone
Outline the 2 types of cell in the thyroid gland and their arrangement
C (clear) cells
Follicular cells - surround hollow follicles by coming together to form the wall of the follicle
Gland is primarily made up of thyroid follicles, surrounded by C cells
Thyroid gland is made up of partly thyroid follicles - what do the centre of these follicles contain
Colloid = sticky glycoprotein matrix that stores thyroid hormone and thyroglobulin until needed to be released
What do thyroid follicles store
2-3 months supply of thyroid hormones
Function of C (clear) cells - a component of the thyroid gland
secrete calcitonin (Regulates Ca2+ - lowers plasma Ca2+)
Functions of follicular cells (4)
produces enzymes needed to synthesise thyroid hormones
produces thyroglobulin (protein rich in tyrosine)
surrounds hollow follicles
concentrates iodide from plasma and transports it into the colloid of thyroid follicles
What class of hormones are thyroid hormones
amine hormones
-as they’re derived from tyrosine
How do be obtain tyrosine and iodide
diet
How do iodide ions in the blood enter follicular cells in the thyroid gland to aid the formation of thyroid hormones (2)
- transport is coupled to what ion
- one entered into the follicular cell, where do the iodide ions next get transported to
via a Na+/I- transporter (symport)
-iodide needs to be coupled to Na+ to enable the follicular cells to take up iodide against a conc. gradient
then I- is transported across the follicle and out of it into the colloid via a pendrin transporter
Iodide enters the follicular cells from the plasma via a Na+/I- transporter (symport)
- what is the difference between the way Na+ and I- enters the cell
Na+ just enters down conc. gradient
I- is transported into cell against a conc. gradient
Outline the steps by which thyroxine (T4) and tri-iodothyronine (T3) are formed
- wha produces the enzymes and other substances needed for TH synthesis
- where are the above transported to
- what does the enzyme produced above catalyse
- addition of one iodide ion to tyrosine produces
- addition of a second iodide ion to tyrosine produces
- T3 is formed by combination of
- T4 is formed by combination of
Follicular cells produce enzymes and thyroglobulin needed to make thyroid hormones
The enzymes and thyroglobulin are packaged into vesicles and exocytosed into the colloid (in the centre of the follicle)
These enzymes catalyse the ADDITION OF IODIDE IONS (that were transported into the colloid via the Na+/I- symporter and pendrin transporter) to tyrosine residues on the THYROGLOBULIN molecule
- Addition of one iodide ion to tyrosine –> MIT (monoiodotyrosine)
- Addition of a second iodide ion to MIT –> DIT (diiodotyrosine)
-Then…
MIT + DIT –> triiodothyronine (T3)
or
DIT + DIT –> tetraiodothyronine aka thyroxine (T4)
Where in the thyroid gland are thyroid hormones synthesised and stored
Colloid (centre of thyroid follicle)
Since, thyroid hormones are stored in the colloid, how are they secreted into the ECF
- how do they get into follicular cell from colloid
- what happens once it’s transported into the follicular cell
- how does TH get into plasma from follicular cell
When stimulated by thyroid stimulating hormone (produced by the AP), portions of the colloid are endocytosed back into the follicular cell in a vesicle
Within the follicular cell, the vesicles then fuse with proteolytic enzymes that cleave the thyroglobulin to release the thyroid hormone
Thyroid hormones are lipid soluble so pass across the membrane into the plasma
- sometimes transport protein (such as MCT transporter) may be involved in moving it into the plasma
Although thyroid hormones are amine hormones derived from tyrosine, why do they not objectively belong to this class
because amine hormones are usually hydrophilic/lipophobic
but T3 and T4 are hydrophobic
Is most of T3 and T4 circulating in the plasma free or protein bound
99.8+ is bound to thyroxine binding globulin
Ratio of T3:T4 circulating in the plasma
- 3nmoles/l : 100nmoles/l
- so about 50x more total T4
- most TH circulates in the form of protein bound T4
Although there’s 50x more total T4 in the plasma than T3, which is more physiologically active + why
T3 because TH receptors inside cells have higher affinity for T3; also there’s more unbound T3 than T4
-90% of TH binding to TH receptors is by T3
How is T4 converted into T3
T4 is deiodinated to T3 by deiodinase enzymes
- some is deiodinated in the plasma and the rest is deiodinated inside target tissue
What external stimuli trigger TRH and subsequent TH release (2)
Cold
Exercise
What hormones inhibit TH release (2)
Somatostatin (GHIH) - inhibits TSH release from AP
Glucocorticoids - inhibit TSH and conversion of T4 to T3
Location of T3/T4 receptors
In the nucleus of target cells
Functions of thyroid hormone (T3) - not T4 has TH receptor has much higher affinity for T3; T4 needs to be converted to T3 to have an effect (6)
raises metabolic rate and promotes thermogenesis,
increases gluconeogenesis
net increase in proteolysis
net increase in lipolysis
stimulates GH receptor expression –> further promoting growth, esp LONE BONE GROWTH
promotes brain development in utero
How does thyroid hormone raise metabolic rate and promote thermogenesis
typically through promoting pointless (FUTILE) cycles of simultaneous catabolism and anabolism
Causes of hyperthyroidism (2)
Graves disease (autoimmune disease)
Thyroid adenoma - hormone secreting thyroid tumour
Pathophysiology of graves disease
TSH mimicking antibodies bind to TSH receptors and continually activate the thyroid gland –> overproduction of TH
So strong negative feedback on TRH and TSH
Thyroid gland hypertrophies as well because of overstimulation by these antibodies
Effects of hyperthyroidism + subsequent symptoms (4)
Increased metabolic rate + heat production –> weight loss/heat intolerance
Increased protein catabolism –> muscle weakness/weight loss
Altered nervous system function –> hyperexcitable reflexes + psychological disturbance
Exaggerated CVS function - increased HR –> increased CO
Causes of hypothyroidism (3)
Hashimoto’s disease - autoimmune attack of thyroid gland
Deficiency in dietary iodine
Idiopathic
Effects of hypothyroidism + subsequent symptoms
Decreased metabolic rate and heat production –> weight gain/cold intolerance
Disrupted protein synthesis –> brittle nails/thin skin
Altered nervous system function –> slow speech, hyporeflexia
Reduced CVS function –> slow HR
What is a goitre
Enlarged thyroid gland
Why does the thyroid gland enlarge in hypothyroidism as well as hyperthyroidism
TSH secretion is increased because of low levels of thyroid hormones so continually stimulating thyroid gland, but thyroid gland can’t respond to make thyroid hormone because deficient in iodine so TH levels remain low but gland just keeps getting stimulated and enlarges
Define euthyroid
Normal thyroid function
What type of epithelial cells line thyroid follicles
Cuboidal
