The Thyroid Gland Flashcards
IS the thyroid gland visible or palpable in health?
No
What does the thyroid gland make?
Thyroid hormones - T3 and T4
T3 = Trii-odo-thyronine T4 = Thyroxine
What 2 cell types are in the thyroid gland?
C/Clear cells
Follicular cells
What do C cells do?
Secrete calcitonin
What do follicular cells do?
Support thyroid hormone synthesis by making the enzymes that make thyroid hormones
Also makes thyroglobulin - a large protein rich in tyrosine residues
Also actively concentrate’s iodide from plasma and transport it to the colloid where it combines with tyrosine to make thyroid hormones
How much supply of TH does the thyroid follicles hold?
2-3 months
Where are the follicular cells?
Surrounding the hollow follicles
What is the colloid?
The hollow space in the centre of sphere
What happens to the enzymes and thyroglobulin made by follicular cells?
They are packaged into vesicles are exported from follicular cells into the colloid
Where is tyrosine and iodide derived from?
The diet
How is T4 - Thyroxine made?
2 tyrosines and 4 Iodides
How is T3 - triiodothyronine made?
2 tyrosines and 3 Iodides
What catalyses the thyroid hormone synthesis reaction and where is it found?
Thyroid peroxidase located on the apical membrane of the follicular cells
How does iodide enter the follicular cells?
Via Na+/I- transported (symport)
The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient
How is iodide transported into the colloid?
Pendrin transporter
What inhibits iodide transport into thyroid gland?
Thiocyanates
What are Thiocyanates? Where do they come from?
Compounds formed from detoxification of cyanide
Common origin is cigarette smoke
What catalyses the addition of iodide to tyrosine residues on the thyroglobulin molecule?
Thyroid peroxidase
and Thyroglobulin itself
How exactly is T3 - triiodothyronine made?
Iodine is added to tyrosine to make monoiodotyrosine and again to make Diiodotyrosine
DIT and MIT are reacted together t make T3
How exactly is T4 - Thyroxine made?
DIT and DIT reacted together
What happens in the thyroid gland in response to TSH?
Thyroglobulin + TH complex is endocytosed into the follicular cells
Once in the follicular cells they form vesicles containing proteolytic enzymes that cut the thyroglobulin to release the thyroid hormones
How do T3 and T4 pass throught the cellular membrane?
They are both hydrophobic/lipophilic so pass through the membrane into the plasma where they bind to plasma proteins
What plasma protein does thyroid hormones mainly bind to?
Thyroxine-binding globulin
Does T3 or T4 circulate in the plasma?
Both do
What is movement of TH from colloid to plasma influenced by?
TSH
Where is TSH released from?
Pituitary gland
Why does T4 have a longer half life than T3?
Thyroxine binding globulin (TBG) has a higher affinity for T4 than T3 releasing it slowly into the plasma
Can bound hormone exert inhibitory effects on TSH and TRH so stop TH release?
No - only free hormone can
Out of T3 and T4, which one is more numerous and by how much?
T4 - ~100 nmoles/L
T3 - ~2.3 nmoles/L
These values are totals and not free hormones
What binds more to receptors inside cells, T3 or T4? What does this mean about each hormones physiological activity?
T3 does 90% of the binding as receptors have a much higher affinity for it -
This means even though T4 is more numerous, T3 is more physiologically active
How is T4 converted to T3?
Deiodinase enzymes deiodinate T4 - 50% in the plasma and the rest inside cells
What does deiodinase activity depend on?
How in demand T3 is to tissues
Explain the pathway of TH release.
Stimulus such as cold, pregnancy or exercise causes hypothalamus to release TRH onto AP gland which releases TSH onto thyroid which stimulates release of TH
What hormones inhibits TH release?
Glucocorticoids and somatostatin
How does Glucocorticoid inhibit TH?
Inhibits TSH and conversion of T4 to T3
How does somatostatin inhibit TH?
Inhibits TSH
Thyroid hormone functions?
Change transcription and translation to alter protein synthesis
Increases metabolic rate and promotes thermogenesis
Increases hepatic gluconeogenesis
Net increase in proteolysis and lipolysis
Critical for growth and foetal brain development
How does TH influence growth?
Has anabolic effects
Has permissive effects on GH
TH/Iodine deficiency in a pregnant mother causes?
Congenital hypothyroidism
What the causes for hyperthyroidism?
Grave’s disease
Thyroid Adenoma
Is graves common?
Yes
Pathophysiology behind Graves?`
Antibodies produced that bind and mimic TSH and continually activate the thyroid gland.
Increased release of TH switches off TSH release from anterior pituitary so [TSH]plasma very low
Thyroid gland may be 2-3x normal size due to hyperplasia.
Hyperactivity of cells also apparent
Is thyroid adenoma common?
No - its rare
What is a thyroid adenoma?
A hormone secreting thyroid tumour
Hyperthyroidism symptoms and why these occur?
Increased metabolic rate and heat production = weight loss and heat intolerance
Increased protein catabolism = muscle weakness/weight loss
Altered nervous system function = hyperexcitable reflexes and psychological disturbances
Elevated CV function - TH is permissive to adrenaline B receptors = Increased HR and contractile force, high output and cardiac failure
Causes of hypothyroidism?
Hashimoto’s - autoimmune
Deficiency in dietary iodine
Idiopathic
How much iodine is needed per year?
Only 50mg
Hypothyroidism symptoms and causes of symptoms?
Weight gain and cold intolerance = decreased metabolic rate and head production
Brittle nails/think skin = disrupted protein sysnthesis
Slow speech/reflexes and fatigue = alterned NS function
Slow HR and weak pulse = impaired CV function
What is goitre?
Enlargement of thyroid gland
What can cause goite?
Hypo and hyper-thyroidism as well as other thyroid pathologies
What causes goitre?
Increased trophic action of TSH on thyroid follicular cells (hypothyroidism)
Overactivity as a result of an autoimmune disease (Grave’s disease/hyperthyroidism)
What is a 1y disorder?
Dirrect effect on endocrine gland making the end hormone
What is a 2y disorder
Pituitary issue
What is a 3y disorder?
Hypothalamic issue
