Chapter 10 endocrine system - thyroid

Question 1

Thyroid gland

Answer 1

• synthesizes and release T3 (triiodothyronine) and T4 (thyroxine)
• both regulate protein synthesis, membrane bound enzymes
• stiulate mitochondrial oxidation
• t3 + t4 both regulate fetal and infant brain development + childhood growth
• overall t3 more potent than t4

T3 + T4 synthesis
1) thyroid follicular cell traps inorganic iodide and oxidizes it into idoine

• Fish (such as cod and tuna), seaweed, shrimp, and other seafood, which are generally rich in iodine
• Dairy products (such as milk, yogurt, and cheese) and eggs, which are also good sources of iodine
• Iodized salt, which is readily available in the United States and many other countries*

2) iodine then binds to tyrosine residues (of thryoglobulin) to form monooiodotyrosine (MIT) and diiodotyrosine (DIT)
3) then either of the two DIT molecules couple to form T4 or MIT couples with DIT to form T3
4) T4 production exceeds T3 production in thyroid gland /// T4 is converted to t3 in periphery

t3 + t4 release/transportation:
* Thyroglobulin (Tg) acts as a substrate for the synthesis of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), as well as the storage of the inactive forms of thyroid hormone and iodine within the follicular lumen of a thyroid follicle. (that’s why thyroglobulin is proteolyzed to release t3 and t4 into circulation)
* only 20% of circulating t3 is secreted by the thyroid (remainder is from degradation of t4)
* thyroxine-binding globulin (TBG) and prealbumin carry most t3 and t4 molecules in the blood (protecting the hormones from degradation)
* ONLY few t3 and t4 are physiologically active
* feedback mech at hypothalamus, pituitary gland, and thyroid gland inhibit or stimulate t3 and t4 production when free thyroid hormone levels are too high or too lower

Question 2

Hyperthyroidism

Answer 2

1) nervousness
2) weakness
3) heat intolerance
4) sweating
5) weight loss
6) warm thin skin
7) exophtalmos (proptosis) – bulging or protruding eyeball or eyeballs
8) loose stool
9) also known as grave’s disease when goiter ( overall enlargement of the thyroid, or it may be the result of irregular cell growth that forms one or more lumps (nodules) in the thyroid) and ocular signs are present

etiology
* Thyroid stimulating AB’s bind to TSH receptors, causing release of thyroxine

labs
* inc’d serum t4 and t3 levels and increased radioiodide uptake

strategy
* control condition with drugs (propylthiouracil or methimazole) for one year then partial resection of thyroid gland

thyroid storm –
* observed in hyperthyroid pts at time of thyroidectomy (surgical trauma causes instant release of thyroid hormones)
* OR in hyperthyroid pt with sepsis

effects:
1) high fever
2) irritabiilty
3) delerium
4) tachycardia
5) diarrhea/vomtiing
6) hypotension
7) coma may develop

tx for thyroid storm —
* Iv glucose and saline
* vitamin B
* glucocorticoids

Question 3

hypothyroidism

Answer 3

features
1) fatigue/weakness
2) cold intolerance
3) hoarseness
4) constipation
5) cold dough like skin
6) thick tongue
7) bradycardia
8) excessive menstural bleeding
9) anemia
10) cretinism in childhood (severely stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones)

etiology
1) primary hypothyroidism via surgery, radioiodine ablation, thyroiditis
2) secondary hypothyroidism – hypofunction of pituitary or hypothalamus

findings
* low concentration of t4 in serum (T4 is produced by the thyroid gland in much greater amounts than T3, around 90% more. This is because when T4 reaches organs and body tissue, it’s converted into T3. So T4 is basically a stepping stone required for T3)
* uptake of radioactive iodide is low b/c thyroid store of iodine are not being used to produce thyroid hormones
* pituitary hormone, TSH – high in primary hypothyrodism and low in secondary hypothyroidism

strategy
* replaced therapy w/ purified or synthetic thyroid hormones

myxdema coma
chornic, severe hypothyrodism results in respiratory depression, hypothermia and stupor
* frequently fatal

Question 4

methimazole (tapazole)
Propylthiouracil (PTU)

hyperthyroidism

Answer 4

• inhibits transformation of inorganic iodine to organic iodine (thyroxine cannot be formed w/o organic form)
• also inhibit iodotyrosine coupling!
• no clinical effects for several days
• Propylthiouracil – same as above but also blocks conversion of t4 to t3 in peripheral tissue

indication
* control hyperthyrodism until sx
* long term drug treatment to avoid surgery
* about 1/2 of pts will remain euthyroid (normal fx’ing) if durg is withdrawn after prolonged use

effects
* temporary hypothyrodism (tx with tyorxine)
* agranulocytosis
* hyperplastic thyroid (enlarged?..
* NOT given to women who are pregnant within 3 yrs —-> damages thyroid of fetus

Question 5

iodine/odide

hyperthyroidism

Answer 5

• inhibit release of thyroxine from thyroid gland
• effects faster (1-3 days) but weaker than methimazole or PTU
• useful for only 2 weeks — gland adapats and resumes thyroxine secretion

indication
* adjunctive therapy used in conjunction with drugs listed above
* provides rapid relief in severely ill patients
* used to devascularize thyroid gland prior to thyroidectomy

effects
* folliculitis (common skin condition that happens when hair follicles become inflamed)
* fever

Question 6

Proporanolol

relieve sx of hyperthyrodism

Answer 6

• b adrnergic reeptor antgonist
• suppress tachycardi + other catecholamine effects

indications
* emergent preparation of hyperthyroid pt for surgery
* thyrotoxicosis in pregnancy
* thyroid storm

effects
* CNS sedation + depression
* suppression of failing heart

Question 7

levothyroxine (t4) [synthroid, levothroid]

hypothyroidism

Answer 7

• replace normal serum levels of t4 and t3 (t4 is converted into t3 by deioidination in periphery)

indication
* DOC for hypothyroidism

effects
* no toxicity @ replacement concentration
* OD causes hyperthyroidism

Question 8

liothyronine (t3)

hypthyrodism

Answer 8

replaces t3

inidcation
* hypothyroid patients w/ difficulty absorbing levothyroxine

effects
* no toxicity @ replacement concentration
* OD causes hyperthyroidism

Question 9

liotrix

hypothryodism

Answer 9

• replace t4 + t3

indication
* when conversion of t4 to t3 is low (myxedema coma — Myxedema coma is defined as severe hypothyroidism leading to decreased mental status, hypothermia, and other symptoms related to slowing of function in multiple organs. It is a medical emergency with a high mortality rate.)
* liotrex may be more usefl than levothyroxine

effects
* no toxicity @ replacement concentration
* OD causes hyperthyroidism