Physiology 76: Thyroid Metabolic Hormones

Front 1

The thyroid makes T4 and T3. Which is made more? Which is more potent?

Back 1

93% T4, 7% T3, but T3 is much stronger

Front 2

Function of TSH

Back 2

stimulate T4/T3 production in thyroid gland

Front 3

What is found inside the follicle cells of the thyroid gland?

Back 3

colloid containing thyroglobulin

Front 4

Thyroxine (T4) production requires what ion?

Back 4

iodine

Front 5

What is iodide trapping?

Back 5

process of concentrating iodide in the follicle cells for T4/T3 production; sodium/iodide transporter (NIS) pumps both Na/I into the cell

Front 6

TSh has what effect on iodine trapping?

Back 6

increases iodine trapping so more T4/T3 can be made; increases activity of NIS pump

Front 7

What is pendrin?

Back 7

moves iodine into the follicle space (brings a Cl into the cell)

Front 8

What is thyroglobulin?

Back 8

created in the follicular epithelium, it is a glycoprotein that contains the tyrosine needed for T4/T3 production; thyroglobulin stores T4/T3 also

Front 9

!st step in T4/T3 production once iodide reaches the follicular space?

Back 9

oxidation of iodide into iodine by peroxidase and hydrogen peroxide

Front 10

Blockage of peroxidase enzyme has what effect on T4/T3 synthesis?

Back 10

decreases; peroxidase is needed to oxidize iodide and begin T4/T3 formation

Front 11

What is organification of thyroglobulin?

Back 11

binding iodine with tyrosine in the thyroglobulin to form monoiodotyrosine (T1) or di-iodotyrosine (T2)

Front 12

Organification of thyroglobulin is assisted by which enzyme?

Back 12

thyroid peroxidase enzymes

Front 13

Once T1 & T2 are made, how are T4/T3 made?

Back 13

coupling of T1 + T2 = T3, T2+T2 = T4

Front 14

When synthesis of T4/T3 stops, why can the effects be seen for months after?

Back 14

excess T4/T3 is stored in thyroglobulin

Front 15

How do T4/T3 enter circulation from the thyroglobulin?

Back 15

pinocytosis from follicle into cell, cleaved from thyroglobulin by proteases, diffuse into blood

Front 16

What does the deiodinase enzyme do?

Back 16

removes iodine from unused T1/T2 to be recycled

Front 17

Once in the blood, how are T4/T3 delivered to tissues?

Back 17

bound to thyroxine-binding-globulin and albumin

Front 18

Once the thyroid hormones are delivered to the cell, why is there a delay before we see a response?

Back 18

they act on DNA hormone response elements to regulate gene transcription; this takes time until an effect is seen in the cell

Front 19

Where are the receptors for thyroid hormone located?

Back 19

in the nucleus

Front 20

Once a thyroid hormone binds its receptor in the nucleus, how does it regulate gene transcription?

Back 20

forms heterodimer with retinoic acid receptor (RXR) and binds hormone responsive element to increase transcription of metabolic components

Front 21

What effect does T4/T3 have on mitochondria?

Back 21

increases # and activity; but could be the result of

Front 22

T4/T3 have what effect on Na/K pumps?

Back 22

increase their activity

Front 23

T4/T3 has what effect on cholesterol, triglycerides, lipids levels in the blood?

Back 23

decreases; T4/T3 make cholesterol stay in bile/feces due to creating more LDL receptors

Front 24

T4/T3 has what effect on free fatty acids in the blood?

Back 24

increases; not sure why, maybe because it is using them for more energy since the metabolic rates are so high

Front 25

What is basal metabolic rate?

Back 25

amount of energy needed by the body to perform most basic functions of the cells; thyroid hormone increases the activity of most cells so more energy is needed for them to maintain function

Front 26

Thyroid hormone has what effects on the circulatory system?

Back 26

increases heart rate, strength, cardiac output, BP, respiration

Front 27

T4/T3 have what effect on GI tract?

Back 27

increased mobility = diarrhea

Front 28

T4/T3 has what effect on sleep?

Back 28

high energy usage makes patients with hyperthyroidism very tired

Front 29

T4/T3 has what effect on insulin release?

Back 29

increases; more glucose needs to get into the cells for energy/metabolic processes

Front 30

TSH binds to its receptors to up-regulate T4/T3 production. What type of receptor is used by TSH?

Back 30

G-protein receptor with cAMP mechanism

Front 31

What is TRH?

Back 31

thyrotropin releasing hormone; from the hypothalamus to regulate TSH secretion

Front 32

What type of receptor/mechanism does TRH use to stimulate TSH?

Back 32

G-protein receptor with DAG, IP3, phospholipase mechanism

Front 33

Cold temperatures have what effect on TSH, T4/T3?

Back 33

levels increase as TRH from the hypothalamus increases

Front 34

How does thiocyanate prevent T4/T3 formation?

Back 34

decrease iodide trapping by out-competing the iodide in the NIS transporter

Front 35

What causes a goiter to form?

Back 35

excess TSH levels

Front 36

Why does thiocyanate lead to goiter formation?

Back 36

thiocyanate = no iodide trapping = no T4/T3 = high TSH levels due to feedback loop trying to get T4/T3 levels up = goiter

Front 37

How does propylthiouracil prevent T4/T3 formation?

Back 37

blocks peroxidase enzyme = less iodine binding tyrosine = less T1/T2 formation = less T4/T3

Front 38

High iodide levels have what effect on thyroid activity?

Back 38

decrease; too much iodide trapping occurs so it slows down due to feedback

Front 39

What is Grave's disease?

Back 39

form of Hyperthyroidism caused by antibodies that mimic TSH, leading to excess T4/T3

Front 40

What levels of TSH would we expect in a patient with Grave's disease?

Back 40

VERY low; these patients have antibodies that are stimulating excess T4/T3 production so TSH will be low due to feedback

Front 41

What is exophthalmos?

Back 41

eyes that protrude from the orbit; seen in hyperthyroidism due to edema of the orbit

Front 42

What is Hashimoto's disease?

Back 42

autoimmune disorder that attacks the thyroid gland = hypothyroidism

Front 43

Why does iodide deficiency cause a goiter?

Back 43

low iodide = less T4/T3 made = more TSH secretion to try to increase T4/T3 levels = goiter. excess TSH leads to goiter by increasing thyroglobulin secretion, swelling the thyroid gland

Front 44

Why would a deficit in the de-iodinase enzyme lead to a goiter?

Back 44

this enzyme recycles the unused iodide for future T4/T3 production. if we didn't have this enzyme there would be less T4/T3 made due to a possible iodide deficit. this would cause high TSH =goiter

Front 45

What is myxedema?

Back 45

facial swelling in patients with low thyroid activity

Front 46

Why does low thyroid hormone lead to atherosclerosis?

Back 46

T4/T3 up-regulate LDL receptors to remove it form the blood. low T4/T3 = low LDL receptors - more LDL in the blood = more foam cells - plaques

Front 47

What is Cretinism?

Back 47

low body growth due to low T4/T3 in children (maybe even congenital lack of thyroid gland)