The Thyroid Gland

Question 1

what shape are thyroid hormones?

Answer 1

shaped like a butterfly, lies across trachea at base of larynx, one of larger endocrine glands, (15-20g), but neither visible nor palpable in health.

Question 2

what are the 2 active forms of thyroid hormones?

Answer 2

T3 (triiodothyronine) and T4 (thyroxine).

Question 3

what are the 2 cell types of thryoid glands>

Answer 3

C (clear) cells

2. Follicular cells

Question 4

what are the function of C cells

Answer 4

secrete calcitonin (Ca2+ regulating hormone).

Question 5

what is the function of follicular cells

Answer 5

support thyroid hormone synthesis and surround hollow follicles.

Question 6

what are thyroid follicles?

Answer 6

spherical structures whose walls are made of follicular cells

Question 7

what is the centre of thyroid follicles filled with?

Answer 7

colloid = sticky glycoprotein matrix. Contain 2-3 mths supply of TH.

Question 8

what do follicular cells manufacture?

Answer 8

the enzymes that make thyroid hormones as well as thyroglobulin, a large protein rich in tyrosine residues.

Question 9

finish the sentence:

the enzymes and thyroglobin are packaged into vesicles and exported from the foliicular cells into the…

Answer 9

colloid

Question 10

follicular cells actively concentrate …. from the plasma and transport it into the collide

Answer 10

iodide

Question 11

what happens to the concentrated iodided that is transported into the colloid?

Answer 11

combines with the tyrosine residues to form the thyroid hormones

Question 12

wht are tyrosine and iodide derived from?

Answer 12

diet

Question 13

how doess iodide enter the follicular cells from the plasma via?

Answer 13

Na/I transporteer

Question 14

how is iodide then transported into the colloid?

Answer 14

pendrin transporter

Question 15

what happens in response to TSH?

Answer 15

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

Question 16

are T3 and T4 lipid soluble or water soluble?

Answer 16

lipid soluble

Question 17

what is the significance of T3 and T4 being lipid soluble?

Answer 17

pass across the follicular cell membrane into the plasma where they bind to plasma proteins, mainly thyroxine-binding globulin. Transporter proteins may also be involved in this process as rare mutations in this protein cause major disruption to TH balance.

Question 18

where do T3 and T4 circulate?

Answer 18

in the plasma

Question 19

how is the movement of TH from colloid to plasma triggered/controlled?

Answer 19

under the influence of TSH released from the pituitary

Question 20

what does TSH stimulate?

Answer 20

the follicular cells to endocytose colloidal thyroglobulin. When not stimulated, the thyroid hormones are stored in the colloid.

Question 21

does thyroxine binding globulin have a high or low affinity for T4?

Answer 21

high which means it releases it slow into the plasma

Question 22

is it more T3 or T4 that circulates in the form of protein bound T4?

Answer 22

T4

Question 23

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

Answer 23

T3

Question 24

the TH receptor has a much higher affinity for

Answer 24

T3 than T4

Question 25

how is T4 deionised to T3?

Answer 25

by deiodinase enzymes

Question 26

what are the functions of thyroid hormone>

Answer 26

raises metabolic rate and promotes thermogenesis, typically through promoting futile cycles of simultaneous catabolism and anabolism.

increase hepatic gluconeogenesis, although no effect on BG providing pancreas is releasing adequate insulin (therefore not a counter regulatory hormone)

net increase in proteolysis

net increase in lipolysis

critical for growth (lack of TH results in retarded growth)

essential for brain development in utero (maternal iodine deficiency = congenital hypothyroidism/cretinism)

Question 27

what are the causes of hyperthyroidism?

Answer 27

Graves Disease (common) - antibodies produced that bind mimic TSH and continually activate the thyroid gland. Increased release of TH switches off TSH release from anterior pituitary so [TSH]plasma very low. Thyroid gland may be 2-3x normal size due to hyperplasia. Hyperactivity of cells also apparent.

Thyroid Adenoma (rare ) - hormone-secreting thyroid tumour

Question 28

what are the symptoms of hyperthyroidism?

Answer 28

. Increased metabolic rate and heat production
 weight loss/ heat intolerance
2. Increased protein catabolism
 muscle weakness/weight loss
3. Altered nervous system function
hyperexcitable reflexes and psychological disturbances
4. Elevated cardiovascular function.TH is permissive to epinephrine, receptors
 increased HR/contractile force, high output, cardiac failure

Question 29

what are causes of hypothyroidism?

Answer 29

Hashimoto’s Disease - autoimmune attack of thyroid gland
Deficiency in dietary iodine – only 50mg/year(!) required but many areas of the world soil has insufficient quantities. Main source of dietary iodine was table salt which was enriched with iodine. But no longer in the UK! Milk, Fish, seafood and seaweed are good sources.
Idiopathic – no known cause, may be linked to thyroiditis.

Question 30

what are symptoms of hypothyroidism

Answer 30

Decreased metabolic rate and heat production
 weight gain/cold intolerance

2. Disrupted protein synthesis
    brittle nails/thin skin
3. Altered nervous system function
    slow speech/reflexes, fatigue
4. Reduced cardiovascular function
    slow heart rate/weaker pulse