The Thyroid Gland

Question 1

Where does the thyroid gland lie?

Answer 1

Across the base of the trachea

Question 2

What is the main hormones produced by the thyroid gland?

Answer 2

T3 (triiodothyronine) and T4 (thyroxine)

The thyroid hormones

Question 3

What kinds of cells does the thyroid gland contain?

Answer 3

C (clear) cells

Follicular cells

Question 4

What is the role of clear cells?

Answer 4

Produce calcitonin

Question 5

What is the role of follicular cells?

Answer 5

Support thyroid hormone synthesis and surround hollow follicles

Question 6

What are thyroid follicles?

Answer 6

Spherical structures whose falls are made of follicular cells
Centre is filled with colloid

Question 7

What is colloid?

Answer 7

sticky glycoprotein matrix

Question 8

What do follicular cells make?

Answer 8

Enzymes to make thyroid hormones + thyroglobulin

Question 9

What happens to the thyroid enzymes and thyroglobulin?

Answer 9

Packaged into vesicles and exported from follicular cells into the colloid

Question 10

Apart from making thyroglobulin + enzymes what else do the follicular cells do?

Answer 10

Actively concentrate iodide from the plasma + transport it into colloid where it combined with tyrosine residues to make thyroid hormones

Question 11

Where are tyrosine and iodide derived from?

Answer 11

The diet

Question 12

What is thyroglobulin?

Answer 12

A large protein rich in tyrosine residues

Question 13

What is T4 composed of?

Answer 13

2 tyrosine + 4 iodide

Question 14

What is T3 composed of?

Answer 14

2 tyrosine + 3 iodide

Question 15

What enzyme catalyses the synthesise of thyroid hormones from the tyrosine residues?

Answer 15

Thyroid perioxidase (located on apical membrane of follicular cells)

Question 16

How does iodide enter the follicular cells from the plasma?

Answer 16

Via a Na/I symport

coupling to Na enables follicular cells to take up iodide against a conc. gradient

Question 17

How is iodide then transported to the colloid?

Answer 17

Via the pendrin transporter

Question 18

What inhibits iodide transport into the thyroid gland?

Answer 18

Thiocynates, compounds formed from detoxification of cyanide (common origin is cigarette smoke)

Question 19

Enzymes are exocytosed into the colloid along with thyroglobulin catalyse what reaction?

Answer 19

Addition of iodide to tyrosine residues
Addition of one iodine to trysoine –> monoiodotryosine (MIT)
Addition of a second iodine –> Diiodotryosine)

MIT + DIT –> T3
DIT + DIT –> T4

Question 20

What does TSH stimulate in the thyroid follicles?

Answer 20

For portions of colloid to be taken into the follicular cell by endocytosis
Within the cells they form vesicles which contain proteolytic enzymes that cut thyroglobulin to release the thyroid hormones

Question 21

What happens after the thyroid hormones are released from thyroglobulin?

Answer 21

T3/T4 are lipid soluble so cross follicular membrane into plasma + bind to plasma proteins, mainly thyroxine binding globulin and then circulate in the plasma

Question 22

What % of T3/T4 is free in the plasma?

Answer 22

0.2% - very highly protein bound

Question 23

Why does T 4 have a higher half life than T3?

Answer 23

Thyroxine binding globulin has a particularly high affinity for T4 so releases it slowly into the plasma

Question 24

What hormone can exert an inhibitor effect on TSH and TRH?

Answer 24

Only free hormone

Question 25

Which of T3 and T4 is more abundant in the plasma?

Answer 25

T4

Question 26

Which of T3 and T4 binds to TH receptors inside cells more?

Answer 26

T3 - TH receptor has much higher affinity for T3 so it is more physiologically active than T4

Question 27

Around half the T4 in plasma is what?

Answer 27

Deiodinated (with the remaining fraction being deiodinated inside target cels) This is the process by which T4 is transformed into T3 y deiodinase enzymes

Question 28

What things increase TRH release?

Answer 28

Cold, stress, pregnancy

Question 29

What things reduce thyroid hormone levels?

Answer 29

Glucocorticoids inhibit TSH and conversion of T4 to T3 Somatostatin inhibits TSH

Question 30

How do thyroid hormones exert their action?

Answer 30

Bind to nuclear receptors inside target cells where they alter transcription and translation to alter protein synthesis

Question 31

What are the roles of thyroid hormones?

Answer 31

Raises metabolic rate, promotes thermogenesis Increases hepatic gluconeogenesis Net increase in proteolysis Net increase in lipolysis Critical for growth --> anabolic, stimulates GH receptor expression Required for foetal brain development

Question 32

Give two examples of things that may cause hyperthyroidism

Answer 32

Grave's disease | Thyroid adenoma

Question 33

What is grave's disease?

Answer 33

Antibodies produced that mimic TSH and continually activate thyroid gland High TH, Low TSH Thyroid gland may be 2-3x larger

Question 34

What is a thyroid adenoma?

Answer 34

Rare hormone secreting thyroid tumour

Question 35

What are the symptoms of hyperthyroidism?

Answer 35

Increased metabolic rate + heat production --> wt loss, heat intolerance Increased protein catabolism --> muscle weakness, wt loss Altered nervous system function --> hyperreflexia, psychological disturbances Elevated CV function, TH permissive to epinephrine, B receptors --> increased HR/contractile force, high output, cardiac failure

Question 36

Give examples of things that cause hypothyroidism

Answer 36

Hashimoto's Iodine deficiency Idiopathic, maybe linked to thyroiditis

Question 37

What is Hashimoto's disease?

Answer 37

Autoimmune attack of thyroid gland

Question 38

What are good sources of iodine?

Answer 38

Fish, seafood, seaweed

Question 39

What are the symptoms of hypothyroidism?

Answer 39

Decreased metabolic rate + heat production --> wt gain, cold intolerance Disrupted protein synthesis --> brittle nails/thin skin Altered nervous system --> slow speech/reflexes, fatigue Reduced CV function --> slow HR, weak pulse

Question 40

What is a goitre?

Answer 40

Enlargement of the thyroid gland

Question 41

What things may cause a goitre?

Answer 41

Increased trophic action of TSH on follicular cells (hypothyroidism) Over-activity due to autoimmune disease (Grave's)