Endocrinology: Thyroid

Question 1

2 major hormones of the thyroid gland

Answer 1

Thyroxine (T4)

Triiodothyronine (T3)

Question 2

function of thyroid hormones

Answer 2

increase metabolic rate of cells in the body

Question 3

hormone that controls secretion of thyroid gland

Answer 3

TSH from anterior pituitary gland

Question 4

93% of metabolically active hormone secreted by thyroid is

Answer 4

Thyroxine

Question 5

How do thyroxine and triiodothyronine differ if they have the same function?

Answer 5

Intensity and speed of effect

Question 6

Compare and contrast thyroxine and triiodothyronine

Answer 6

Both have same function of increasing metabolic rate

Triiodothyronine is in less concentration, but is 4 times more potent than thyronine and persists for a much short time. the opposite is true for thyronine

Question 7

Type of epithelium lining follicles of thyroid gland

Answer 7

single Cuboidal epithelial

Question 8

Substance in follicles and its composition

Answer 8

colloid

mostly made of the glycoprotein thyroglobulin

Question 9

Why does thyroid have almost greater blood supply than any other area of the body?

Answer 9

Its blood flow is almost five times its weight each minute

Question 10

Iodine is required to form

Answer 10

thyroxine

Question 11

Mechanism of iodide trapping

Answer 11

ATPase on basal membrane creates sodium potassium gradient. Sodium flows back down its concentration gradient into the cell together with Iodine with the help of the symport sodium-iodie symporter.
Process of concentrating iodine in the cell is called iodide trapping

Question 12

Function of pendrin

Answer 12

A counter-transporter on apical membrane of cuboidal cells that transports iodine into the follicular space in exchange for chlorine

Question 13

Effect of increasing TSH concentration on iodide trapping

Answer 13

increases rate of iodide trapping

Question 14

Thyroglobulin is synthesized by

Answer 14

the cuboidal cells

Question 15

Importance of thyroglobulin

Answer 15

To which iodine binds and within which iodine used to make T3 and T4

Question 16

Define organification

Answer 16

binding of iodine to thyroglobulin

Question 17

Importance of peroxidase system at the apical cell membrane

Answer 17

It oxidizes iodine so that it binds to thyroglobulin. Without this system, production of thyroid hormone falls to 0

Question 18

To which molecule does iodine bind to in the thyroglubulin to form thyroid hormones?

Answer 18

Tyrosine amino acids

Question 19

Number of tyrosine amino acids in a thyroglobulin molecule

Answer 19

70

Question 20

Why is peroxidase action on iodide important?

Answer 20

Oxidizes the iodide to iodine, the only form of iodide which can bind to tyrosine amino acids

Question 21

Shortly list steps of T3 and T4 formation from iodide

Answer 21

Tyrosine –> Monoiodotyrosine –> Diiodotyrosine

Monoiodotyrosine + Diiodotyrosine = T3

Diiodotyrosine + Diiodotyrosine = T4

Question 22

Location of the peroxidase enzyme and its strategic importance

Answer 22

Located in apical membrane or attached to it. Importance is that when iodide is being pumped into the follicles, it is oxidized almost immediately into iodine so that by the time it enters the follicles it is ready to bind to thyroglobulin

Question 23

Why it’s difficult to detect hypothyroidism immediately at its onset.

Answer 23

Because it has enough thyroid hormone in store for 2 to 3 months so no physiological symptoms are produced

Question 24

2 Mechanisms of release of thyroid hormones.

Answer 24

1.Pinocytosis of colloid together with its thyroglobulin found to thyroid hormones
Pinocytotic vesicles fuse wit lysosome vesicles containing proteases that cleave thyroglobulin
Thyroid hormones released
Cross basal membrane into adjacent capillaries

2. Binds to megalin on the apical membrane
   Taken in by endocytosis
   Edocytotic vesicles move via transcytosis to basal membrane
   Thyroid hormones released via exocytosis into adjacent capillaries

Question 25

How is majority of iodine recycled in the body?

Answer 25

about 3/4 of iodinated tyrosine does not form T3 and T4, remaining as monoiodotyrosine and diiodotyrosine

deiodinase enzyme cleaves the iodine from these thyroglobulin molecules to make it available again for making more thyroid hormones

Question 26

True or false: The thyroid hormone that reaches cells is thyroxine since it’s secreted in greater amount than triiodothyronine.

Answer 26

False: Although thyroxine is secreted in greater amounts, it is converted to triiodothyronine so most tissues get T3 instead of T4.

Question 27

Which thyroid hormone binds more strongly to intracellular proteins? plasma proteins? intracellular thyroid receptors?

Answer 27

Thyroxine binds more strongly to intracelullar proteins.

Triiodothyronine binds more strongly to intracellular thyroid receptors.

Thyroxine binds more strongly to plasma proteins.

Question 28

Name the receptor thyroid hormone receptor complex binds to at the thyroid hormone response element.

Answer 28

retinoid X receptor

Question 29

State non-genomic effects of thyroid hormone

Answer 29

activation of cAMP system
regulation of ion channels
regulation of oxidative phosphorylation (of iodide)

Question 30

cAMP mechanism of TSH

Answer 30

binds to TSH receptors on basal membrane of thyroid cells 
activation of adenylyl cyclase 
formation of cAMP
phosphorylation of protein kinases 
effects of TSH observed

Question 31

What class of hormones based on chemical structure does TSH fall into? and TRH?

Answer 31

TSH –> a glycoprotein

TRH –> amide

Question 32

Structure of TRH

Answer 32

made of 3 amino acids
glutamate
histidine
proline

Question 33

True or false: When hypophyseal blood portal system is blocked, TSH secretion falls to zero because no more TRH is reaching it

Answer 33

False: It falls close to zero, but not completely

Question 34

Molecular mechanism for how TRH exerts its effects on anterior pituitary cells

Answer 34

Binds to TRH receptors
Activation of phospholipase C
Cleavage of phosphatidyl inositol to form IP3 and DAG
= TSH release

Question 35

Difference in mechanisms of TRH and TSH

Answer 35

TRH uses phospholipase second messenger system while TSH uses cAMP system

Question 36

Factors affecting amount of TSH and TRH secreted

Answer 36

Cold - exposure of animal to cold increases TRH and TSH secretion = high metabolism due to high heat produced

Emotions that cause excess stimulation of sympathetic nervous system (fear, anxiety, stress) = increase in TSH and TRH, but the thyroid hormone released in turn goes to inhibit TSH resulting ultimately in low levels of TSH secretion

Question 37

Effect of cold and emotions on the thyroid secreting activity of thyroid gland after hypothalmic stalk is cut

Answer 37

No effect since TRH can’t reach anterior pituitary = reduced TSH = reduced thyroid

Question 38

General differences in the clinical presentations, diagnosis, and treatment of hypothyroidism versus hyperthyroidism

Answer 38

Hyperthyroidism

• weight loss (due to increased BMR), sweating, muscle weakness, anxiety, diarrhea, intolerance to heat
• usually presents with high levels of Thyroid hormone and low levels of TSH in blood but depends on the cause of hyperthyroidism
• treatment is by partial removal thyroid gland, radioactive iodine to destroy thyroid tissue

Hypothyroidism

• weight gain, muscle sluggishness, reduced cardiac output, atherosclerosis (due to buildup of cholesterol in blood), constipation, sleepiness and slow thinking
• usually presents with high TSH levels, but low thyroid hormone levels *varies
• treatment is by administration of T3 tablets

Question 39

How is a patient with hyperthyroidism prepared for surgery?

Answer 39

several weeks before operation, antithyroid drug like propylthiouracil is administered to return BMR to normal
Then 1-2 weeks, inorganic iodides are given to reduce the size of gland + its blood supply to reduce amount of bleeding