Thyroid Gland - CB

Question 1

What are the biologically active forms of thyroid hormone (TH)?

Answer 1

T3 (triiodothyronine) and T4 (Thyoxine)

Question 2

Fact: the thyroid is highly vascularized

Answer 2

It also has a large store of TH

Question 3

What stimulates the synthesis and release of TH?

Answer 3

TSH (Thyroid stimulating hormone) from ant. pituitary

Question 4

What cells synthesize and release TH?

Answer 4

follicular epithelial cells

Question 5

What cells synthesize/release calcitonin?

Answer 5

parafollicular cells

Question 6

What is the amino acid precursor of TH, T3, T4? What else is needed for syn?

Answer 6

tyrosine bound to TG (thyroglobulin)

iodide (only hormone that need an essential trace element)

Question 7

Is reverse T3 bioloically active?

Answer 7

no, just regular T3

Question 8

Is T4 or T3 preferentially synthesized/released?

Answer 8

T4

Question 9

Where is iodide stored? how is it obtained?

Answer 9

thyroid gland associated with Thyroglobulin, from Diet, a 2Na+/I cotransporter (symporter)

Question 10

Fact: TSH is under negative feedback by TH

Answer 10

I should have been an engineer…

Question 11

Describe the synthesis and storage of T3/T4?

Answer 11

1. TG is made in ER, packed into vessicles in the golgi and exocytosed into the lumen of the folice
2. Iodide enteres thyrocyte via 2Na+/I cotransporter and exits into the lumen via I/Cl antiporter
3. Iodide is oxidized in lumen by thyroid peroxidase and added to a tyrosine on TG
4. Organification: the binding of Iodine to tyrosine one TG
   - MIT: monoiodotyrosine (1 Iodine)
   - DIT: diiodotyrosine (2 Iodines)
   Thyroid peroxidase couples MIT and DIT forming T3 and 2 DITs forming T4
5. The mature TG complex is endocytosed back into the folliculocyte and stored as colloid (Mature TG has MIT>DIT>T4>T3)
6. TSH stimulates Colloid proteolysis relseasing T3 and T4 from TG and it exits into the blood via the basolateral membrane. The MIT and DIT reenter the synthetic pool

Question 12

What is circulating TH bound to?

Answer 12

thyroid-binding protein mainly (Not TG!)
also transthyretin and albumin
small amount it free (bioavailable) ( it can enter target cell nuclei and bind)

Question 13

What is T4 metabolized into?

Answer 13

T3 and reverse T3 (rT3)

Question 14

What from of TH is most biologically active?

Answer 14

T3, T4 is called a prohormone

Question 15

What is TH broken down into?

Answer 15

Thyronine

Question 16

What is used to treat hypothyroidism? Why?

Answer 16

T4, longer half life, metabolized into the more active T3

Question 17

What form of TH can enter the cell? Where are its targets?

Answer 17

Unbound, in the nucleus, the receptor binds T3 then DNA

Question 18

What tissues have TH receptors?

Answer 18

ALL

Question 19

What does TH do?

Answer 19

regulates (increases) the metabolism of carbs, proteins and lipids

also acts with Growth Hormone and somatomedins to promote bone growth, ossification, bone maturation

CNS development

Question 20

How are cytoplasmic T3 and T4 ratio maintained?

Answer 20

5’/3’ monodeiodinase converts T4 to T3 so they are equal

Question 21

Describe the kinetics of TH? (Onset speed and duration)

Answer 21

Slow onset, long lasting (weeks)

Question 22

Fact: in hypothyroidism, bone age is less than actual age

Answer 22

excessive treatment of hypothyroidism with T4 can cause osteoporosis (post menopausal women)

Question 23

TH deficiency in infants can have what effect?

Answer 23

Growth and Mental retardation

Question 24

What are some system effects of TH?

Answer 24

growth, CNS maturation, Increased BMR, Increased metabolism, increased cardiovascular output

Question 25

Decreased BMR can be associated with ___thyrodism?

Answer 25

hypo

Question 26

Treatment for hyperthyroidism?

Answer 26

Propanolol, Beta adrenergic receptors are overly expressed causing some symptoms. This drug is a beta-blocker

Question 27

Activators and inhibitors of TH release?

Answer 27

Activators: TRH (hypothalamus) and TSH (ant. pituitary)

inhibitors: T3/4 (neg feedback), somatostatin (all act indirectly by inhibiting TSH release )

Question 28

How is thyroid disease diagnosed?

Answer 28

serum TSH
High TSH means hypothyroidism (lack of negative feedback by T3/4)

low TSH means hyperthyroidism (too much neg feedback by T3/4)