thyroid gland physiology Flashcards
how many glands does the thyroid gland have
2
how are the 2 glands joined together
by narrow portion of the gland called the isthmus
where is the gland located
over the trachea just below the larynx
what are the major thyroid secretory cells
follicular cells
what do the follicular cells form
follicle
what are the functional units of thyroid
follicles
what is the inner lumen filled part of the follicle called
colloid
what does the colloid store
thyroid hormone
is the colloid intracellular or extracellular
extracellular
what is most of the colloid made of
thyroglobulin
what 2 hormones do follicular cells produce
- tetraiodine thyronine (T4) - tri-iodothyronine (T3)
what secretory cells are interspersed between follicles
C cells
what do C cells secrete
calcitonin
what does calcitonin play a role in
calcium metabolism
what are taken up from the blood but the follicular cels
- tyrosine - iodine
what is tyrosine
amino acids
how do we acquire iodine
dietary intake
what is dietary iodine reduced to
iodide
SYNTHESIS AND STORAGE OF THYROID HORMONE
SYNTHESIS AND STORAGE OF THYROID HORMONE
where does thyroid hormone synthesis take place
within the colloid
where is thyroglobulin produced
by the endoplasmic reticulum of the thyroid follicular cell
what happens to tyrosine containing thyroglobulin once it is exported from follicular cells
goes into the colloid by exocytosis
how does iodide get into the colloid
via the iodide pump
how does the iodide pump work
symporter driven by Na+ / K+ pump
what happens in the iodide pump
transports Na+ into the follicular cells and I- into the cell
where is iodide mostly in the body
in the thyroid cells
what happens to iodide in the follicular cells
is it oxidised to ‘active’ iodide
how is iodide oxidised
by thryoperoxidase (TPO)
how does active iodide get into the colloid
exits through a channel in the luminal membrane to enter the colloid
once iodide enters the colloid what happens
TPO attaches iodide to a tyrosine
what odes attachment of iodide to tyrosine form
monoiodotyrosine (MIT).
what does attachment of two iodine to tyrosine form
di-iodotyrosine (DIT)
what happens once MIT and DIT are formed
coupling process occur which forms the thyroid hormones
1 MIT + 1 DIT forms
tri-iodothyronine (T3)
1 DIT + 1 DIT =
tetraiodothyronine (T4) = thyroxine
can 2 MIT molecules join
no
where do T3 and T4 remain
stored in the colloid until they are split off and secreted
SECRETION OF THYROID HORMONE
SECRETION OF THYROID HORMONE
what happens to secrete thyroid hormones from the colloid
the follicular cells internalise a portion of the thyroglobulin complex by phagocytosing part of the colloid
what happens when the colloid is internalised
T3 and T4 are split by lysosomes into free T3 and T4 as well as MIT and DIT
are thyroid hormones lipophilic
yes so can pass the membrane of follicular cells and into the blood
are MIT and DIT of value
no
what happens to MIT and DIT
follicular cells have an enzyme called iodinate that removes the iodine from MIT and DIT
what happens to the free iodine
they are recycled for synthesis of more hormone
what is the majority of T3 and T4 transported by
thyroxine-binding globulin
is most T3 or T4 free or bound
bound
is bound T3 and T4 able to bind to receptors
no only the free form
T4 CONVERTING TO T3
T4 CONVERTING TO T3
what is secreted more T4 or T3
T4
what is more potent T3 or T4
T3
what happens to most of T4
it is converted into T3
how is T4 activated to T3
it is stripped of one of its iodide outside the thyroid gland
where is T4 converted to T3
liver and kidneys
what happens when thyroid hormone crosses the plasma membrane
binds to a nuclear receptor bound to the thyroid-response element of DNA - this alters transcription of mRNA
what does the receptor have a higher affinity for T3 or T4
T3
does thyroid hormone affect all body cells
yes it can in one way or another
how dose thyroid hormone affect metabolic rate
increases it
what does thyroid hormone do to catecholamines
stimulates them
effect of thyroid hormone on cardiovascular status
increase heart rates and force of contraction therefore increase cardiac output
is thyroid hormone essential for normal growth
yes
does thyroid hormone stimulate growth hormone secretion
yes
does excess thyroid produce excess growth
no
does thyroid hormone play an essential role in normal development of nervous system
yes
REGULATION OF THYROID HORMONE
REGULATION OF THYROID HORMONE
what its het most important regulator of thyroid hormone secretion
thyroid stimulating hormone
what produces thyroid stimulating hormone
the anterior pituitary
what does thyroid stimulating hormone increase
cAMP in the thyrotropes
what is every step of thyroid hormone synthesis stimulated by
TSH
other than enhancing thyroid hormone secretion what else does TSH have a role in
maintaining the structural integrity of the thyroid gland
what happens to the thyroid gland in the absence of TSH
atrophies (decreases in size)
how does excess TSH affect the thyroid gland
hypertrophy and hyperplasia i
what turns on TSH secretion
thyrotropin releasing hormone
what turn off TSH secretion
thyroid hormone
how does TRH function via
the IP3/DAG/ Ca2+ pathway
what is the only factor that increase TRH secretion
exposure to cold in newborn infants
what factors inhibit TSH and thyroid hormone secretion
stress starvation infection
ABNORMALITIES IN THYROID FUNCTION
ABNORMALITIES IN THYROID FUNCTION
what are the 2 main thyroid disorders
- hypothyroidism - hyperthyroidism
how can hypothyroidism occur
- primary failure of thyroid gland - secondary to a deficient TRH, TSH or both - inadequate dietary supply of iodine
symptoms of hypothyroidism
- reduced BMR - poor tolerance to cold - gain excessive weight - easily fatigued - slow, weak pulse - slow reflexes - diminished alertness - slow speech - poor memory - puffy appearance in face and hands (myxoedema)
what develops if someone has hypothyroidism from both
cretinism
what is cretinism characterised by
- dwarfism - mental retardation
treatment of hypothyroidism
replacement therapy by administering thyroid hormone
what is the most common cause of hyperthyroidism
Graves disease
what is Graves disease
autoimmune disease in which the body produces excess thyroid stimulating immunoglobulin (TSI)
what does TSI stimulate
secretion and growth of the thyroid
does TSI affect the negative feedback loop
nope it has no effect on negative feedback
symptoms of hyperthyroidism
- elevated BMR - increased heat production - excessive perspiration - poor tolerance to heat - weight loss - muscle weakness - palpitations - irritable - tense - anxious
prominent features of Graves disease
- exophthalmos
what is exopthalamos
bulging eyes - the eye may bulge so much that they cannot close their eyes
treatment of hyperthyroidism
- antithyroid drugs - surgical removal of a portion of the over secreting thyroid gland - administration of radioactive iodine
what happens when radioactive iodine is administered
destroys thyroid glandular tissue
GOITER
GOITER
what is a goiter
enlarged thyroid gland
is a goitre palpable
yes
can goitre be present in hypothyroidism and hyperthyroidism
yes
when is a goitre present in hypothyroidism
- thyroid gland failure - lack of iodine
why is there a goitre in hypothyroidism
as its a negative feedback system there is little thyroid hormone so TSH is being produced this increases the size of the thyroid gland
if there is still TSH why may there not be thyroid hormone
due to lack of iodine or enzymes
why is there a goitre in hyperthyroidism
- excessive TSH secretion - Graves disease (TSI promotes growth)
when would there be hyperthyroidism but no goitre
thyroid tumour
why is there no goitre in thyroid tumour
as there is excessive amounts of T3 and T4 it inhibits TSH so there is no input to promote growth of the thyroid
look at diagram of anatomy and histology
diagram
diagram
