The thyroid gland Flashcards
What is the name for a greatly enlarged thyroid?
Goitre
What is TSH?
a pituitary dimeric glycoprotein composed of an alpha and a beta subunit.
The alpha subunit is the same as:
TSH
LH
FSH
hCG
TSH binds to a G-protein couple receptor on thyroid follicle epithelial cells, which activates both:
- Adenylate Cyclse = cAMP/PKA dependant pathway
- Phospholipase C = PI turnover = DAG and IP3
iodine is actively taken up from the blood into follicular cells and transporter into the lumen. in the lumen the enzyme ________ ________ converts iodine to free iodine.
Thyroid peroxidase
Thyroglobulin contains a high content of what?
tyrosine
Iodine atoms are incorporated into?
Tyrosine residues
MIT stands for:
mono-iodotyrosine = one iodine per tyrosine
DIT stands for:
Di-iodotryrosine = two iodine per tyrosine
DIT + DIT =
T4
MIT + DIT =
T3
Two ____ do NOT couple
MIT
what do lysosomal cells in the follicular cells do?
break down the iodinated thyroglobulin liberating free T3 and T4
How does T3 and T4 enter the blood?
Diffuse through the plasma membrane of the follicular cells
Inactive DIT and MIT are rapidly?
de-ionated and the released iodine is recycled back to the lumen for further use
follicular cells take up a portion of the _____ in the lumen (________)
Colloid
Endocytosis
What does TBG stand for?
Thyroxine binding globulin
What does TBG circulate bound to?
T3 AND T4, because they have poor solubility in water
What does TBG prevent?
Urinary secretion of T3 andT4
TBG ____ against ____ changes in the ______ functions
buffers
acute
thyroid
before exerting a biological effect what must T3/T4 do?
dissociate from TBG
what is secreted 90%; T3 or T4?
T4, T3 is secreted 10%
What is more potent T3 or T4?
T3 is 3 fold more potent than T4
how is most of secreted T4 activated?
by the removal of an iodine in target peripheral tissues
T3 and T4 bind to
intracellular thyroid hormone receptors
the receptor that T3 and T4 bind to is a:
HORMONE REPSONSIVE TRANSCRIPTION FACTOR, T3 is more active as it binds with a higher affinity
what are the effects of thyroid hormone?
-Increases basa metabolic rate (BMR)
raised CO2 consumption
CO2 production
Heat production
-Cardiovascular system
increased hear force and contraction
-Nervous system
increases activity of sympathetic nervous system
enhances the sensitivity to catecholamines
-Growth and maturation
embryo development
CNS development
Linear growthHow does a goitre come about?
hypertrophy of thyroid gland caused by excessive stimulation by TSH
associated with certain forms of hypo or hyper thyroids
often associated with autoimmune disease
Effects of Hypothyroidism in Adults:
weight gain cold intolerance lethargy depression puffiness of skin and muscles sluggish reflexes muscular weakness reduced pulse rate and cardiac output
Effects of hypothyroidism in foetus/child
brain damage - permanent if not treated early enough
shortness
obesity
mental retardation
What is done to prevent foetus/child hypothyroidism?
routine neonatal screening for elevated TSH.
High TSH indicated low T3/T4 due to reduced -ve feedback on hypo. pit
Iodine Deficiency Disorder (IDD) is the worlds most prevalent cause of?
brain damage
how many people worldwide does it effect?
740 million, 200 million with goitre
IDD is easily avoided by?
use of iodised cooking salt/supplementary iodine in foods
What is the treatment for HYPOthyroidism?
lifelong replacement of T4 (adult ~150ug/day), unless due to chronic dietary iodine deficiency in which case increased supplementary iodine is given in diet.
What are the causes of hypothyroidism?
- Primary failure of thyroid gland
- Secondary due to hypothalamic or anterior pituitary failure
- Secondary due to autoimmune damage to gland
- Secondary due to chronic lack of dietary iodine
What is HYPERthyroidism?
excess thyroid hormone
what are the symptoms of hyperthyroidism?
weight loss nervousness heat intolerance high cardiac output hand tremors eyeball protusion
50% of hyperthyroidism are due to _________
Grave’s disease
What causes graves disease?
autoimmune disease
thyroid stimulating Ab that mimics TSH:
Primary = hyper secreting thyroid tumour
secondary= excessive hypothalamic/anterior pituitary secretion
What is the treatment for hyperthyroidism?
anti-thyroid drugs that interfere with thyroid hormone synthesis
surgical resection of thyroid
thyroid ablation using radioactive iodine
Elevated ___ inhibits the release of ___ and ___ = _______ __________
T3
TSH
TRH
negative feedback
