The thyroid gland

Question 1

What are the 2 physiologically active forms of thyroid hormone

Answer 1

T3 (triiodothyronine)

T4 (thyroxine)

Question 2

What are the 2 main cell types of the thyroid gland

Answer 2

C (clear cells)

Follicular cells

Question 3

What do c cells secrete

Answer 3

Calcitonin (ca2+ regulating hormone)

Question 4

Function of follicular cells

Answer 4

support thyroid hormone synthesis ie T3+T4

manufacture enzymes that make thyroid hormones as well as thyroglobulin

and they line the follicles

Question 5

What is the l of the thyroid follicle filled with?

Answer 5

Colloid - a sticky glycoprotein matrix

Question 6

What 2 things are packaged into vesicles and exported from the follicular cells into the colloid?

Answer 6

Enzymes and thyroglobulin

Question 7

What happens to the enzymes and thyroglobulin that follicular cells produce?

Answer 7

They are packaged into vesicles and exported from the follicular cells into the colloid

Question 8

How do follicular cells form thyroid hormones

Answer 8

They actively concentrate iodide from the plasma and transport it into the colloid where it combines with the tyrosine residues to form the thyroid hormones.

Question 9

Where are tyrosine and iodide derived from

Answer 9

diet

Question 10

How does iodide enter the follicular cells

Answer 10

via a Na+/I- transporter

The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient

Question 11

How does iodide enter the colloid

Answer 11

Via the pendrin transporter

Question 12

More specific description of how T3 and T4 are produced

Answer 12

enzymes exocytosed into the colloid and thyroglobulin catalyse the oxidation of iodide to iodine

iodine + tyrosine residues on the thyroglobulin

Addition of one iodine to tyrosine = MIT (monoiodotyrosine).

Adding a second iodine = DIT (diiodotyrosine).

So MIT+DIT = triiodothyronine (T3)

DIT+DIT = tetraiodothyronine (T4)

Question 13

What catalyses the reactions of tyrosine into T3 or T4

Answer 13

thyroid peroxidase

Question 14

What is the response to thyroid stimulating hormone in follicular cells

Answer 14

they take back up portions of the colloid by endocytosis

within the follicular cells they form vesicles which contain proteolytic enzymes that cut thyroglobulin to release thyroid hormones

Question 15

Why can T3 and T4 pass across the follicular cell membrane?

Answer 15

they are lipid soluble

Question 16

What happens once T3 and T4 are in the plasma?

Answer 16

they bind to plasma proteins mainly thyroxine-binding globulin (99.8%) (rare mutations in this protein cause major disruption to TH balance)

Question 17

Movement of thyroid hormones from the colloid to the plasma is under what influence

Answer 17

under the influence of thyroid stimulating hormone released from the pituitary

TSH stimulates the follicular cells to endocytose colloidal thyroglobulin

Question 18

What is one reason why T4 has a longer half life than T3

Answer 18

Thyroxine Binding Globulin (TBG) has a particularly high affinity for T4, releasing it only slowly into the plasma.

Question 19

Why is T3 3-5 times more physiologically active than T4?

Answer 19

The TH receptor has a much higher affinity for T3

90% of TH binding to TH receptors inside cells is T3

Question 20

What happens when T4 is deiodinated?

Answer 20

it becomes T3

around half the T4 is deiodinated in the plasma and the rest is deiodinated inside target cells

Question 21

General function of thyroid hormones

Answer 21

increase the basal metabolic rate

affect protein synthesis

help regulate long bone growth

neural maturation

increase the body’s sensitivity to catecholamines (such as adrenaline) by permissiveness

Question 22

What factors increase the tonic release of TSH (3)

Answer 22

Cold
Exercise
Pregnancy

Question 23

What do somatostatins inhibit

Answer 23

TSH

Question 24

What do glucocorticoids inhibit

Answer 24

TSH and conversion of T4 to T3

Question 25

What is the effect of Thyroid hormone binding to nuclear receptors in their target cells?

Answer 25

They change transcription and translation to alter protein synthesis

Question 26

Result of altering protein synthesis

Answer 26

raises metabolic rate and promotes thermogenesis increase hepatic gluconeogenesis, although no effect on BG providing pancreas is releasing adequate insulin net increase in proteolysis net increase in lipolysis Growth - lack of TH results in retarded growth

Question 27

Grave's disease

Answer 27

Common disease where antibodies are produced that mimic Thyroid Stimulating Hormone and continually bind to and activate the thyroid gland. Increased TH release from gland switches off TSH release from the anterior pituitary therefore [TSH] plasma becomes very low Thyroid gland may be 2-3x normal size due to hyperplasia. Hyperactivity of cells also apparent.

Question 28

What is a thyroid adenoma

Answer 28

A rare hormone-secreting thyroid tumour

Question 29

Explanation of symptoms of hyperthyroidism

Answer 29

weight loss/ heat intolerance - due to increased M.R and heat production muscle weakness/weight loss - due to increased protein catabolism hyperexcitable reflexes and psychological disturbances - altered nervous system function increased HR/contractile force, high output, cardiac failure - elevated CV function. TH is permissive to epinephrine, B receptors

Question 30

Symptoms of hyperthyroidism

Answer 30

increased HR/contractile force, high output, cardiac failure hyperexcitable reflexes and psychological disturbances muscle weakness/weight loss weight loss/ heat intolerance

Question 31

What is Hashimoto's disease

Answer 31

An autoimmune attack of the thyroid gland Most common cause of hypothyroidism due to deficiency in dietary iodine idiopathic

Question 32

Symptoms of hypothyroidism

Answer 32

weight gain/cold intolerance brittle nails/thin skin Slow speech/reflexes, fatigue slow HR/weaker pulse

Question 33

Define the term Goitre

Answer 33

Significant enlargement of the thyroid gland

Question 34

Why does Goitre occur

Answer 34

May be caused by increased trophic action of TSH on thyroid follicular cells (hypothyroidism) or over-activity as a result of autoimmune disease (Graves Disease)

Question 35

True or false: Goitre can occur in both hypothyroidism and hyperthyroidism

Answer 35

True