The Thyroid Gland

Question 1

Describe the structure of the thyroid gland?

Answer 1

Butterfly shaped, lies across trachea at base of larynx
Not visible or palpable in health
One of the larger endocrine glands

Question 2

What are the 2 active forms of thyroid hormones?

Answer 2

T3 - triiodothyronine
T4 - thyroxine

Question 3

What are the 2 cells in thyroid gland?

Answer 3

C (clear) cells
Follicular cells

Question 4

What are C cells?

Answer 4

Secrete calcitonin - Ca regulating hormone

Question 5

What are follicular cells?

Answer 5

Support thyroid hormone synthesis and surround hollow follicles

Question 6

What are thyroid follicles?

Answer 6

Spherical structures which walls are made of follicular cells
Centre of follicle filled with colloid - sticky glycoprotein matrix

Question 7

How much TH does the colloid contain?

Answer 7

2-3mnths supply

Question 8

What do follicular cells produce?

Answer 8

Enzymes which make thyroid hormones as well as thyroglobulin - protein rich in tyrosine residues
These are secreted into colloid

Question 9

Explain iodide and follicular cells

Answer 9

These cells actively concentrate iodide from plasma and transport it into colloid where it combines with tyrosine residues to form TH

Question 10

How does iodide enter follicular cells?

Answer 10

From plasma via Na/I symport transporter
Allows iodide to be transported against conc. gradient
Into colloid by pendrin transporter

Question 11

What is the role of thyroid peroxidase?

Answer 11

Enzymes exocytose into colloid with thyroglobulin
Catalyses the oxidation of iodide to iodine
Addition of iodine to tyrosine residues on thyroglobulin molecule

Question 12

What is the addition of one iodine and a second iodine to thyroglobulin?

Answer 12

MIT - monoiodotyrosine
DIT - diiodotyrosine

Question 13

What does conjugation of MIT and DIT form?

Answer 13

MIT + DIT - T3 triiodothyronine
DIT + DIT - T4 thyroxine
Conjugation by thyroid peroxidase

Question 14

What happens in response to TSH?

Answer 14

Portions of colloid undergo endocytosis and within these cells they are packaged into vesicles containing proteolytic enzymes that cut thyroglobulin to release TH

Question 15

How do TH travel in plasma?

Answer 15

They are lipophilic so pass across follicular cell membrane and bind to plasma proteins - mainly thyroxine binding globulin

Question 16

Where is TSH released from?

Answer 16

Anterior pituitary

Question 17

Describe circulating TH

Answer 17

More than 99.8% of T3 and 4 circulates in plasma bound to plasma protein - T4 higher affinity so longer half life
Only free hormone exerts an inhibitor effect on TSH and TRH

Question 18

Explain negative feedback control on TH

Answer 18

Free hormone in plasma exerts effects on TSH (anterior pituitary) and TRF (hypothalamus)
Can lower at both these points if too much TH

Question 19

Is T3 or T4 more physiologically active?

Answer 19

T3 as TH receptor has higher affinity for T3 than T4 so 3-5 x more active
90% of TH binding to receptor is T3

Question 20

What happens to T4?

Answer 20

De-iodinated to T3 by deiodinase enzymes
Half de-iodinated in plasma and half inside target cells as TH receptors are in cell

Question 21

What are hypothyroid patients supplemented with?

Answer 21

T4 but have normal T3 levels

Question 22

What can increase tonic release of TRH?

Answer 22

Cold, pregnancy and exercise

Question 23

What hormones are inhibitory for TSH?

Answer 23

Glucocorticoids - inhibits TSH and conversion of T4 to 3
Somatostatin (GHIH) - inhibits TSH

Question 24

What does thyroid hormone bind to and what does this do?

Answer 24

Nuclear receptors in target cells - change transcription and translation to alter protein synthesis

Question 25

What is the function of TH?

Answer 25

Raises metabolic rate and promotes thermogenesis Increases hepatic gluconeogenesis Net increase in proteolysis Net increase in lipolysis Critical in growth and brain development in utero

Question 26

How does TH promote thermogenesis?

Answer 26

Through promoting futile cycles of simultaneous catabolism and anabolism - produces heat and not ATP

Question 27

What are some causes of hyperthyroidism?

Answer 27

Graves Disease Thyroid adenoma - hormone secreting thyroid tumour

Question 28

How does Graves disease cause hyperthyroidism?

Answer 28

Antibodies produced mimic TSH and continually activate thyroid gland Increased release turns off TSH from pituitary so low TSH in plasma Hyperplasia of gland and hyperactivity

Question 29

What are the symptoms of hyperthyroidism?

Answer 29

Weight loss/heat intolerance Muscle weakness/weight loss Hyper excitable reflexes and physiological disturbances Increased HR/contractile force as TH is permissive to epinephrine B receptors

Question 30

What are some causes of hypothyroidism?

Answer 30

Hashimoto's disease - autoimmune attack on thyroid gland Deficiency in dietary iodine Idiopathic

Question 31

What are the symptoms of hypothyroidism?

Answer 31

Weight gain/ cold intolerance Brittle nails and thin skin Slow speech/ reflexes and fatigue Slow heart rate and weaker pulse

Question 32

What is goitre?

Answer 32

Significant of thyroid gland in both hypo or hyperthyroidism Increased trophic action of TSH on thyroid follicular cells in hypo and over activity in graves disease

Question 33

Describe hypothyroidism due to low iodine

Answer 33

Low iodine in diet so T3 and T4 remain low so no negative feedback so increased TRH and TSH released

Question 34

Describe hyperthyroidism due to Graves disease

Answer 34

Thyroid stimulating immunoglobulins make gland enlarge and increase T3 and T4 so strong negative feedback which decreases TSH and TRH but does not effect thyroid