Thyroid Physiology Flashcards Preview

Endocrinology > Thyroid Physiology > Flashcards

Flashcards in Thyroid Physiology Deck (28):
1

What hormones does the thyroid secrete?

T3-triiodothyronine
T4-thyroxine
Calcitonin

2

What is the biologically active thyroid hormone? Why?

free T3
more potent

3

How do thyroid hormones travel in the blood?

Bound to plasma proteins
TBG ( thyroxine binding globulin)
TBPA (thyroxine binding prealbumin)
Albumin

4

What is the most abundant plasma protein transporter for thyroid hormones?

TBG (70 %)

5

What is the most abundant thyroid hormone?

T4

6

What thyroid hormone levels correlate with metabolic state?

Free thyroid hormone levels

7

What are the cellular components of the thyroid gland?

Follicle
Follicular cells (make ring like structure around colloid)
Colloid (tyrosine containing, thyroglobulin filled)
Parafollicular C cells (secrete calcitonin)

8

How are thyroid hormones released from the cell?

Iodine uptake into follicular cells
Move into colloidal cells:
Iodine attached to tyrosine residue from thyroglobulin makes MIT and DIT
(MIT+DIT=T3, DIT+DIT=T4)
Stored in colloid thyroglobulin
Pinocytosis of T3 and T4
T3 and T4 released into bloodstream

9

What is the thyroid hormone axis?

Hypothalamus: TRH secreted (thyrotrophin releasing hormne)
Pituitary: TSH production increased (thyroid secreting hormone)
Thyroid: TSH binds to follicular cells, T3/4 secreted from colloid
Negative feedback from hormones to hypothalamus and pituitary

10

What action do thyroid hormones have on lipid metabolism?

increase fatty acid oxidation
mobilises adipose tissue fat

11

What action do thyroid hormones have on growth?

GHRH requires thyroid hormones for secretion (permissive action)
GH somatomedins (permissive action)

12

What action do thyroid hormones have on CHO metabolism?

increases blood glucose
increases insulin dependant glucose uptake in cells
increase gluconeogenesis and glyconeogenesis

13

What action do thyroid hormones have on thermogenesis?

hormones released in response to cold stress

14

What action do thyroid hormones have on basal metabolic rate?

increases number and size of mitochondria
increased respiratory chain enzymes
increased O2 and ATP hydrolysis

15

What action do thyroid hormones have on foetal/neonatal brain development?

myelinogenesis
axonal growth

16

What action do thyroid hormones have on protein metabolism?

increased protein synthesis

17

What permissive sympathetic actions do thyroid hormones have?

increases response to adrenaline
increases response to sympathetic neurotransmitters
increases no. sympathetic receptors

18

How will change in levels of TBG affect levels of thyroid hormones in blood?

Changes total hormone level
Not free hormone level (weren't bound anyway)

19

What conditions can cause an increase in TBG?

Pregancy
Hep A
Newborn
Chronic active hepititis
Genetics
Heroin usage

20

What conditions can cause a decrease in TBG?

Large dose glucocorticoids
Cushing's
Androgens
Active acromegaly
Nephritic syndrome
genetics

21

How does a change in temperature affect thyroid hormone levels?

Cold temperature
TRH release
TSH release
T3/T4

22

How does stress affect thyroid hormone release?

Inhibits TRH and TSH

23

How do changes in levels of thyroid hormone throughout the day demonstrate a circadian rhythm?

Highest levels at night
Lower levels in am

24

How do hyper/hypo thyroidism affect ovulation?

anovulation

25

What effect does pregnancy have on thyroid hormones?

thyroxine important for development
increased demand on thyroid
plasma protein binding increases

26

How does pregnancy effect hypothyroid patients?

thyroid can't meet extra demand

27

How does hCG effect thyroid function?

Acts like TRH

28

How is TBG effected in pregancy?

increased protein binding
normally fT4 normal
fT4 may be low if hypothyrioid