The thyroid gland Flashcards
What are the 2 physiologically active forms of thyroid hormone
T3 (triiodothyronine)
T4 (thyroxine)
What are the 2 main cell types of the thyroid gland
C (clear cells)
Follicular cells
What do c cells secrete
Calcitonin (ca2+ regulating hormone)
Function of follicular cells
support thyroid hormone synthesis ie T3+T4
manufacture enzymes that make thyroid hormones as well as thyroglobulin
and they line the follicles
What is the l of the thyroid follicle filled with?
Colloid - a sticky glycoprotein matrix
What 2 things are packaged into vesicles and exported from the follicular cells into the colloid?
Enzymes and thyroglobulin
What happens to the enzymes and thyroglobulin that follicular cells produce?
They are packaged into vesicles and exported from the follicular cells into the colloid
How do follicular cells form thyroid hormones
They actively concentrate iodide from the plasma and transport it into the colloid where it combines with the tyrosine residues to form the thyroid hormones.
Where are tyrosine and iodide derived from
diet
How does iodide enter the follicular cells
via a Na+/I- transporter
The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient
How does iodide enter the colloid
Via the pendrin transporter
More specific description of how T3 and T4 are produced
enzymes exocytosed into the colloid and thyroglobulin catalyse the oxidation of iodide to iodine
iodine + tyrosine residues on the thyroglobulin
Addition of one iodine to tyrosine = MIT (monoiodotyrosine).
Adding a second iodine = DIT (diiodotyrosine).
So MIT+DIT = triiodothyronine (T3)
DIT+DIT = tetraiodothyronine (T4)
What catalyses the reactions of tyrosine into T3 or T4
thyroid peroxidase
What is the response to thyroid stimulating hormone in follicular cells
they take back up portions of the colloid by endocytosis
within the follicular cells they form vesicles which contain proteolytic enzymes that cut thyroglobulin to release thyroid hormones
Why can T3 and T4 pass across the follicular cell membrane?
they are lipid soluble
What happens once T3 and T4 are in the plasma?
they bind to plasma proteins mainly thyroxine-binding globulin (99.8%) (rare mutations in this protein cause major disruption to TH balance)
Movement of thyroid hormones from the colloid to the plasma is under what influence
under the influence of thyroid stimulating hormone released from the pituitary
TSH stimulates the follicular cells to endocytose colloidal thyroglobulin
What is one reason why T4 has a longer half life than T3
Thyroxine Binding Globulin (TBG) has a particularly high affinity for T4, releasing it only slowly into the plasma.
Why is T3 3-5 times more physiologically active than T4?
The TH receptor has a much higher affinity for T3
90% of TH binding to TH receptors inside cells is T3
What happens when T4 is deiodinated?
it becomes T3
around half the T4 is deiodinated in the plasma and the rest is deiodinated inside target cells
General function of thyroid hormones
increase the basal metabolic rate
affect protein synthesis
help regulate long bone growth
neural maturation
increase the body’s sensitivity to catecholamines (such as adrenaline) by permissiveness
What factors increase the tonic release of TSH (3)
Cold
Exercise
Pregnancy
What do somatostatins inhibit
TSH
What do glucocorticoids inhibit
TSH and conversion of T4 to T3
What is the effect of Thyroid hormone binding to nuclear receptors in their target cells?
They change transcription and translation to alter protein synthesis
Result of altering protein synthesis
raises metabolic rate and promotes thermogenesis
increase hepatic gluconeogenesis, although no effect on BG providing pancreas is releasing adequate insulin
net increase in proteolysis
net increase in lipolysis
Growth - lack of TH results in retarded growth
Grave’s disease
Common disease where antibodies are produced that mimic Thyroid Stimulating Hormone and continually bind to and activate the thyroid gland.
Increased TH release from gland switches off TSH release from the anterior pituitary
therefore [TSH] plasma becomes very low
Thyroid gland may be 2-3x normal size due to hyperplasia.
Hyperactivity of cells also apparent.
What is a thyroid adenoma
A rare hormone-secreting thyroid tumour
Explanation of symptoms of hyperthyroidism
weight loss/ heat intolerance - due to increased M.R and heat production
muscle weakness/weight loss - due to increased protein catabolism
hyperexcitable reflexes and psychological disturbances - altered nervous system function
increased HR/contractile force, high output, cardiac failure - elevated CV function. TH is permissive to epinephrine, B receptors
Symptoms of hyperthyroidism
increased HR/contractile force, high output, cardiac failure
hyperexcitable reflexes and psychological disturbances
muscle weakness/weight loss
weight loss/ heat intolerance
What is Hashimoto’s disease
An autoimmune attack of the thyroid gland
Most common cause of hypothyroidism due to deficiency in dietary iodine
idiopathic
Symptoms of hypothyroidism
weight gain/cold intolerance
brittle nails/thin skin
Slow speech/reflexes, fatigue
slow HR/weaker pulse
Define the term Goitre
Significant enlargement of the thyroid gland
Why does Goitre occur
May be caused by increased trophic action of TSH on thyroid follicular cells
(hypothyroidism)
or over-activity as a result of autoimmune disease (Graves Disease)
True or false: Goitre can occur in both hypothyroidism and hyperthyroidism
True
