Thyroid Gland Flashcards
what does the thyroid gland consist of?
2 lobes of endocrine tissue joined in the middle by narrow portion of the gland
what is the function of the follicular cells?
line cavities and produce two iodine containing hormones derived from tyrosine (tetraiodothyronine (T4) and triiodothyronine (T3))
how are the follicular cells arranged?
into hollow spheres (functional unit = follicle)
the lumen is filled with colloid
what is the function of colloid?
fill the lumen and serve as extracellular storage site for thyroid hormone
what are the two main kinds of cell found in the thyroid gland?
follicular cells
C cells
what is the function of C cells?
secrete peptide hormone calcitonin
what importance is tyrosine and iodine in the thyroid gland?
basic ingredients for the synthesis, storage, and secretion of thyroid hormones
where do the synthesis steps occur within the colloid?
on thyroglobulin molecules
what are the basic steps of synthesis in the thyroid gland?
1) Tyr-containing thyroglobulin is exported from follicular cells into colloid by exocytosis
2) thyroid captures iodine from blood and transfers it into colloid by iodine pump
3) within colloid, iodine attaches to Tyr
4) coupling process occurs between iodinated tyr molecles to form thyroid molecules
briefly describe the storage methods for hormones the thyroid gland.
thyroid hormones remain in colloid until they are split off or secreted
enough stored to supply the body for several months
what is the process involved in the secretion of hormones from the thyroid gland?
1) follicular cells phagocytize thyroglobulin-laden colloid
2) process frees T3 and T4 to diffuse across plasma membrane and into blood
what is MIT and DIT? what is their significance with T3 and T4?
MIT = tyrosine monoiodinated DIT = tyrosine diiodinated T3 = MIT + DIT T4 = DIT + DIT
What effects do thyroid hormones have on the body? (6pts)
1) main determinant of basal metabolic rate
2) influences synthesis and degradation of carbohydrate,s, fat, and protein
3) increases target-cell responsiveness to catecholamines
4) increases HR and force of contraction
5) essential for normal growth
6) plays crucial role in normal development of NS
what hormones are involved in the feedback system of the hypo-pit-thyroid axis?
1) hypothalamic thyroid releasing hormone (TRH)
2) ant. pit. thyroid stimulating hormone (TSH)
3) T3/T4
what is the feedback system of the hypo-pit-thyroid axis?
1) stress (-ive) or cold in infants (+ive) affects hypo., causing release of TRH
2) TRH acts on ant pit, causing release of TSH
3) TSH acts on thyroid gland to stimulate the release of T3 and T4, which negatively impacts the ant pit
4) T3 and T4 act on the body in various ways
what is hypothyroid? what are symptoms of hypothyroidism?
not enough T3 and T4
symptoms: cold all the time, gain wt, sleepy, frequently stumbling over words
what is hyperthyroid? what are symptoms of hyperthyroidism?
too much T3 and T4
symptoms: racing heart beat, feeling warm, can’t sleep, losing weight
what are 3 causes of hypothyroidism?
1) primary failure in thyroid gland
2) secondary to a deficiency of TRH, TSH, or both
3) inadequate dietary supply of iodine
in the 3 causes of hypothyroidism, which has a goitre present?
1 and 3 - goitre present
2 - goitre not present
what is a health consequence of hypothyroidism?
cretinism - severely shunted physical/mental growth
what is myxedema?
swollen feet/hands and skin is course in appearance?
what is the treatment option of myxedema?
replacement therapy (dietary iodine)
what is the most common cause of hyperthyroidism?
Graves’ disease
what is Graves’ disease?
autoimmune disease where there is an abnormal presence of long-acting thyroid stimulator (LATS)
what causes Graves’ disease?
feedback loop that should be regulating levels of TSH, T3, and T4 is not working because the body is producing a molecule that mimics TSH
body produces thyroid-stumulating immunoglobulins (TSI)
what is characteristic of Graves’ disease?
exopthalmos
what is exopthalmos?
bulging eyes
what are your treatment options for hyperthyroidism?
Surgical removal of portion of the over-secretion thyroid
administration of radioactive iodine
use of antithyroid
what are 2 other causes for hyperthyroidism? when is a goitre present?
1) excess hypothalamic or ant pit secretion (goitre present)
2) hypersecreting thyroid tumour (Goitre not present)
For each of the 3 causes of HYPOthyroidism, do the following hormones increase or decrease:
T3
T4
TSH/TRH (not involved in all causes)
1) decr. T3 and T4; incr. TSH
2) decr. T3 and T4; decr. TRH and TSH
3) decr. T3 and T4; incr. TSH
for the 3 causes of HYPERthyroidism, do the following hormones increase or decrease:
T3
T4
TSH/TRH (no involved in all causes)
1) (Graves) incr. T3 and T4; decr. TSH
2) (hypersecreting hypo/ant pit) incr. T3 and T4; incr TRH and/or TSH
3) (tumour) incr. T3 and T4; decr. TSH
how does the feedback system work with LATS (as in Graves’ disease)?
1) LATS positively impacts thyroid gland which acts on thyroid hormones
2) thyroid hormones negatively impact ant. pit to inhibit secretion of TSH, which prevents stimulation of thyroid gland
why must plasma Ca be closely regulated?
to prevent changes in neuromuscular excitability
what functions does plasma Ca have? (4pts)
1) excitation-contraction coupling in cardiac/smooth muscle
2) stimulus-secreting coupling
3) maintenance of tight junctions between cells
4) clotting of blood
what happens in the case of hypercalcemia?
reduced excitability
what happens in the case of hypocalcemia?
overexcitability of nerves/muscles
severe overexcitability can cause fatal spastic contractions of respiratory muscles
what 3 hormones regulate plasma Ca concentration?
Parathyroid hormone (PTH)
Calcitonin
Vitamin D
What secretes PTH?
parathyroid glands
How does PTH affect Ca?
raises free plasma Ca levels by its effects on bones, kidneys, and intestines
what are 2 other important functions of PTH?
prevents fatal consequences of hypoglycemia
facilitates activation of Vitamin D
What 2 things are secreted by osteoblasts (and their precursors)?
RANK ligand
Osteoprotegerin (OPG)
what is the function of RANK ligand?
act on macrophages/osteoclasts (binds to RANK) to either change macrophages into osteoclasts or suppress osteoclast apoptosis
increase in osteoclast action to the point where it surpasses osteoblast action leads to decrease in bone mass
what is the function of OPG?
Binds to RANK ligand to prevent it from binding to RANK thus preventing its action and loss of bone mass
decrease in osteoclast action to the point where it is surpassed by osteoblast action, leading to an increase in bone mass
What is the basic process of bone formation?
Osteoblast forms osteocyte
osteocyte becomes calcified and turns into bone
what happens when there is a decrease in plasma Ca?
stimulates parathyroid glands to increase PTH, which increases plasma Ca
dominant pathway
what happens when there is an increase in plasma Ca?
stimulates thyroid C cells to increase calcitonin, leading to a decrease in plasma Ca
what is the function of Calcitonin?
secreted in response to increase in plasma Ca. it works to lower plasma Ca levels by inhibiting activity of bone osteoclasts
what is the function of Vitamin D?
stimulates Ca and PO4 absorption from intestines
what requirements are in place before Vitamin D can exert its effect on the intestines?
activated first by the liver and then by the kidneys
what is the relationship between Ca and PO4
inverse relationship
decrease in plasma PO4 causes increase in plasma Ca
what are 3 problems that could arise from Ca metabolism?
PTH hypersecretion (hyperparathyroidism) PTY hyposecretion (hypoparathyroidism) Vit D deficiency
what is characteristic of PTH hypersecretion?
hypercalcemia and hypophosphatemia
what is characteristic of PTH hyposecretion?
hypocalcemia and hyperphosphatemia?
what are the different conditions caused by Vitamin D deficiency that arise in children and adults?
children - rickets
adults - osteomalacia
what is rickets?
no calcification leading to bendy bones
what is osteomalacia?
tendency for bones to become brittle and fragile
