Physiology: Physiology of the Thyroid

Question 1

Describe the effect of thyroid hormones on lipid metabolism

Answer 1

Mobilised fats from adipose tissues, increased fatty acid oxidation in tissues

Question 2

Name three states which increase thyroxine binding globulin levels

Answer 2

Pregnancy, chronic active hepatitis, billiary cirrhosis

Question 3

Which type of de-iodinase breaks down the majority of thyroid hormones?

Answer 3

Type III (D3)

Question 4

Describe the effect of thyroid hormones on growth

Answer 4

Growth hormone releasing hormone (GHRH) production and secretion, and glucocorticoid-induced GHRH release are dependent on thyroid hormones

Question 5

Describe the effect of thyroid hormones on metabolic rate

Answer 5

Increase basal metabolic rate

Question 6

Where are Type II (D2) de-iodinases found?

Answer 6

Heart, skeletal muscle, fat, thyroid, and pituitary

Question 7

Describe the effect of thyroid hormones on the development of foetal and neonatal brain

Answer 7

Myelinogenesis and axonal growth require thyroid hormones

Question 8

Describe the effect of thyroid hormones on the lungs

Answer 8

Increased breathing rate

Question 9

What affect do T3 and T4 have on the hypothalamus and anterior pituitary?

Answer 9

Suppress the production of TRH and TSH - complete the negative feedback loop

Question 10

Describe the structure of a thyroid follicle

Answer 10

Colloid surrounded by cuboidal epithelial cels and a basement membrane

Question 11

Name the organ which produces TSH

Answer 11

Anterior pituitary

Question 12

Name the principle thyroid hormone binding protein

Answer 12

Thyroxine binding globulin (70%)

Question 13

How are the thyroid hormones formed from the colloid?

Answer 13

Iodine moves into the colloid and attaches to tyrosine residues on thyroglobulin to form DIT (T3) and MIT (T4)

Question 14

Name three states which decrease thyroxine binding globulin levels

Answer 14

Large doses of glucocorticoids/Cushing’s, severe systemic illness, chronic liver disease

Question 15

Which thyroid hormone acts as the pro hormone?

Answer 15

T4 - converted to T3 by liver and kidney to become biologically active

Question 16

How do thyroid hormones increase metabolic rate?

Answer 16

Increase number and size of mitochondria, increase oxygen use and rates of ATP hydrolysis, increase synthesis of respiratory chain enzymes

Question 17

Why do most labs measure free T3/4 rather than serum levels?

Answer 17

Metabolic state correlates more closely with the free than with the total concentration of thyroid hormones in the plasma

Question 18

Describe the effect of thyroid hormones on liver tissue

Answer 18

Increased glycogenolysis and gluconeogenesis, decreased glycogenesis

Question 19

What is the colloid in the thyroid follicle?

Answer 19

Iodinated glycoprotein thyroglobulin

Question 20

Describe the effect of thyroid hormones on the heart

Answer 20

Increased HR and force of contraction

Question 21

Which thyroid hormone accounts for ~90% of thyroid hormones secreted?

Answer 21

T4 (thyroxine)

Question 22

What percentage of thyroid hormones are bound to hormone-binding proteins?

Answer 22

More than 99%

Question 23

Describe the effect of thyroid hormones on the central nervous system

Answer 23

Hypothyroidism - slow intellectual functions

Hyperthyroidism - nervousness, hyperkinesis and emotional lability

Question 24

What do the parafollicular cells secrete?

Answer 24

Containing calcitonin-secreting C-cells

Question 25

Name the organ which produces TRH

Answer 25

Hypothalamus

Question 26

Why does thyroid hormone increase breathing rate, heart rate and force of cardiac contraction?

Answer 26

Helps meet increased O2 demand

Question 27

Describe the secretion of T3 and T4

Answer 27

Colloid is enveloped by microvilli on cell surface to form colloid vesicles which fuse with lysosomes; enzymes break down the lysosomes releasing thyroid hormone

Question 28

Which thyroid hormone accounts for ~10% of thyroid hormones secreted?

Answer 28

T3 (triodothyronine)

Question 29

Name the key enzymes involved in degradation of thyroid hormones

Answer 29

De-iodinases

Question 30

What is the function of TSH?

Answer 30

Binds to TSH receptor (GPCR) on surface of thyroid epithelial cells and via cAMP increases production and release of thyroid hormones (T3 + T4)

Question 31

Describe the effect of thyroid hormones common to all cells but especially neurones

Answer 31

Increase metabolic rate, increased glucose uptake

Question 32

Where are Type III (D3) de-iodinases found?

Answer 32

Faetal tissue, placenta, and brain (except pituitary)

Question 33

Where are Type I (D1) de-iodinases found?

Answer 33

Liver and kidney

Question 34

What is the function of TRH?

Answer 34

Stimulates anterior pituitary to produce TSH

Question 35

Name the three types of hormone-binding proteins which thyroid hormones bind to

Answer 35

Thyroxine binding globulin, thyroxine binding pre-albumin, albumin

Question 36

Describe the permissive sympathomimetic action of thyroid hormones

Answer 36

Thyroid hormones increase responsiveness of adrenaline and noradrenaline by increasing the number of receptors

Question 37

Which thyroid hormone is the most potent?

Answer 37

T3 (triodothyronine)

Question 38

Describe the effect of thyroid hormones on adipose connective tissue

Answer 38

Increased lipolysis, decreased lipogenesis

Question 39

How do free thyroid hormones increase BMR of target cells?

Answer 39

Bind to receptor on target cells, translocate into the nucleus and bind to thyroid response elements on target genes to stimulate transcription of these genes

Question 40

Describe the effect of thyroid hormones on carbohydrate metabolism

Answer 40

Increased blood glucose due to stimulation of glycogenolysis and gluconeogenesis, increased insulin-dependent glucose uptake into cells

Question 41

Describe the effect of thyroid hormones on protein metabolism

Answer 41

Increased protein synthesis