Thyroid Hormone

Question 1

relationship between TRH, TSH, and TH

Answer 1

TRH released from hypothalamus –> (+) TSH from pituitary gland

then TSH stimulates TH release from thyroid gland

Question 2

what hormones does the thyroid produce

Answer 2

T3 and T4

both increase the rate of O2 consumption and metabolism adjusted to alteration in heat production, energy need, caloric supply, and environment temperature

Question 3

components of a thyroid hormone

Answer 3

2 tyrosines and an iodine

called iodothyronines

Question 4

reverse T3

Answer 4

unknown function

differs from T3 by only location of 1 of 3 iodines

the alternative product of prohormone T4 and made when less TH action is needed

Question 5

uptake and concentration of iodide with the gland

1st step

Answer 5

actively transported into the thyroid by Na+/I-cotransporter

‘iodide trap’ which maintains a high ratio of free iodide concentration in the gland to the iodide concentration in plasma

iodide deficiency –> TH deficiency regardless if trap mechanism is functional

Question 6

iodination of tyrosine within thyroglobulin

Answer 6

inside the follicle lumen, iodide incorporated into tyrosine molecules at specific sites

enzyme = thyroid peroxidase

results = MIT and DIT

Question 7

coupling of two iodinated tyrosine

Answer 7

thyroid peroxidase

DIT + DIT –> T4

DIT + MIT –> T3

Question 8

ratio of T4:T3

Answer 8

10:1

restricted iodide availability –> increase T3 formation

Question 9

how are iodinated thyroglobulins stored

Answer 9

within the follicle as colloid

Question 10

secretion steps of thyroglobulin

Answer 10

1. transferred from the lumen of the follicle –> endocrine cells by endocytosis
2. lysosomal enzymes release T3 and T4 from thyroglobulin
3. T3 and T4 leave through basement membrane and enter capillary circulation
4. MIT and DIT are rapidly de-iondinated by deiodinase and iodide is reused for new hormone synthesis

Question 11

what thyroid reactions occur in the cell?

interface between cell membrane and the EC colloid space?

Answer 11

in cell = iodide trap, and thyroglobin synthesis

iodination reaction and coupling reaction

Question 12

T4 is prohormone for what

Answer 12

T3

T4 can also produce some intrinsic IC action of its own

Question 13

thyroxin-binding globulin

Answer 13

what T4 and T3 circulate bound to (20%)

rest to transthyretin and albumin

create circulating reservoir of T4

Question 14

which TH is more hormonally active

Answer 14

T3

T4 only 25% as active

thus important to convert T4 –> T3

Question 15

5-monodeiodinase

Answer 15

reaction T4–>T3

Question 16

major targets for TH

Answer 16

liver, kidnet, and skeletal muscle

Question 17

how does TH enter cells

Answer 17

energy dependent transport mechanism

most of the T4 is then converted to T3

both are transported to the nucleus and bind to nuclear receptors
- T3 has higher affinity

interact with DNA to effect transcription

Question 18

Effects of T3 after it increases transcription in target cells (i.e. intracellular effects)

Answer 18

increases in the following

1. sympathetic effect
2. expression Na+/K+ ATPase
3. number of mitochondria
4. respiratory enzymes
5. other enzymes, proteins

** all of these together lead to increase in O2 consumption and metabolic rate

Question 19

Whole body effects of T3

Answer 19

1. increase CO and ventilation –> increase O2 –> increase O2 consumption –> increase metabolic rate –> …. A & B
2. increase food intake and mobilization of endogenous carbs, proteins, and fat –>… A & B

A. increase CO2, ventilation, urea, renal function, thermogenesis, and sweating

B. decrease muscle mass and adipose tissue

Question 20

increased TH production effect on basal metabolic rate (BMR)

Answer 20

increases –> increased thermogenesis

effects of this increased heat production on raising body temperature is counteracting by…
- increases in blood flow, sweating, and ventilation –> which cool the body temperature

Question 21

increasing TH production on the O2 consumption

Answer 21

increases it without increased work

hypothyroid rate = 150mL/min
euthyroid = 250
hyper = 400

Question 22

increasing TH on Na+/K+ ATPase

Answer 22

increases ADP –> leads to increase in mitochondria O2 uptake

also

increases ADP into mitochondria and ATP out by binding to a receptor on mitochondria
= ‘non-genomic effect’ (does not involve gene transcription)

Question 23

increasing TH = catabolic or anabolic?

Answer 23

catabolic

leads to more…rapid drug metabolism, vitamin turnover, and steroid hormone metabolism

Question 24

TH on kidney and kidney function

Answer 24

increases kidney size

and…

renal blood flow, GFR, and increased transport of kidney tubules
–> i.e. increases capacity to reabsorb solutions such as glucose and amino acds

Question 25

TH on heart and cardiac function

Answer 25

increases cardiac stroke volume

increases heart rate, systolic blood pressure, and decreased diastolic blood pressure

increased sarcolemal uptake of Ca2+ –> shortens diastole… which increases the rate at which heart can beat