Thyroid Gland

Question 1

what happens to the thyroid gland during pregnancy/menstruation

Answer 1

increases in size

Question 2

what is the thyroid gland innervated by

Answer 2

CNX

Question 3

what is the average weight of the thyroid gland

Answer 3

20-30g

Question 4

describe a follicle

Answer 4

follicular cells encase the colloid

parafollicular cells are located adjacent to these and lie within the connective tissue

Question 5

what do follicular cells produce

Answer 5

thyroglobulin

Question 6

what do parafollicular C cells produce

Answer 6

calcitonin

Question 7

thyroglobulin

Answer 7

the precursor of T3 and T4

produce by the thyroid follicular cells and stored in the colloid until it is required

Question 8

describe the synthesis of thyroid hormones

Answer 8

• iodide is absorbed from the blood stream into the follicular cells and passes into the colloid
• it attaches to tyrosine residues in the thyroglobulin molecules to form MIT and DIT
• T3 is formed from one MIT and one DIT
• T4 is formed from two DITs

Question 9

what happens in the follicular cells when thyroid hormones are needed

Answer 9

• pinocytosis occurs - pinching off of some thyroid hormone bound to thyroglobulin to bring it into the follicular cells from the colloid
• lysosomes in the follicular cells release the thyroid hromones
• T3 and T4 are released and secreted into the bloodstream

Question 10

which thyroid hormone does the thyroid mainly produce

Answer 10

T4

a reservoir for the more active and main thyroid hormone T3, which it becomes by peripheral conversion, mainly in the liver

Question 11

how are T3 and T4 found in the body

Answer 11

they are hydrophobic/lipophilic so are found bound to plasma proteins:

• TBG - 70%
• TTR
• albumin

Question 12

what state are the thyroid hormones in when they are active

and how does this relate to thyroid function tests

Answer 12

unbound is their active state

therefore, measuring total T3 and T4 gives no reflection of te biologically active molecules

= free T3 and T4 are measured as only free hormone is available to tissues

Question 13

describe the regulation of thyroid hormones

Answer 13

the hypothalamus secretes TRH, which controls TSH release by the thyrotroph cells in the anterior pituitary gland

both TSH and THR release is inhibited when elevated thyroid hormone levels are detected in the blood and provide negative feedback to hypothalamus and anterior pituitary gland

Question 14

describe TSH signalling at follicular cells

Answer 14

TSH binds to a G protein coupled receptor on the follicular cells, causing cellular effects through the conversion of ATP to cAMP

Question 15

what effect can ill health have on hormones

Answer 15

stress inhibits TSH and THR release

low temperatures in babies stimulates TRH release

sensitive to ill health

= thyroid function tests are not reliable in ill health

Question 16

do thyroid hormone levels vary throughout the day?

Answer 16

yes - they exhibit a Circadian rhythm

higher late at night and lowest in the morning

Question 17

how do tissues regulate thyroid level

Answer 17

Serum thyroid levels may not necessarily predict tissue thyroid levels under a variety of physiological conditions. Activity of deiodinase enzymes allows specific tissue levels of T4 and T3.

Question 18

deiodinase enzymes

Answer 18

• subfamily of 3 enzymes that are important in the activation and deactivation of thyroid hormones
• they work by adding/removing an iodine atom from the outer ring
• they contribute to the activation/inactivation of T4 into T3, or rT3 in target cells
• T3 can be converted into inactive T2

Question 19

where are D 1, 2 and 3 usually found

Answer 19

D1 - liver and kidney

D2 - heart and skeletal muscle, CNS, fat, thyroid and pituitary

D3 - foetal tissue and placenta and brain

Question 20

what are the actions of the deiodinase enzymes

Answer 20

• D1 and D2 convert T4 to T3 and increase cellular thyroidactivity.
• D3 reduces cellular thyroid activity by converting T4 to rT3.

Question 21

thyroid hormone receptor on tissues

Answer 21

transmembrane receptor

has alpha and ß isoforms, and each of these has several different variants eg TRß1 and TRß2

Question 22

what would a defect in thyroid hormone receptor on tissues cause

Answer 22

features of hypothyroidism despite the normal production of thyroid hormones

Question 23

which thyroid hormone receptors are expressed where in the body

Answer 23

can help predict where the symptoms will arise

Question 24

what is RTHa characterised by

Answer 24

tissue specific hypothyroidism with near normal thyroid function tests

mutation in THRa leads to defective signalling through THRa receptor

Question 25

what is RTHß characterised by

Answer 25

hyperthyroidism with increased levels of TH and TSH

(ß receptors are have a mainly inhibitory action in the hypothalamic pituitary axis and so inc production and inc circulation)

Question 26

calcitonin

Answer 26

released by parafollicular C cells in the thyroid gland

responsible for modulating blood calcium levels in conjunction with PTH

Question 27

action of calcitonin

Answer 27

lowers blood calcium in 3 ways:

• inhibit osteoclast mediated breakdown of bones
• stimulate osteoblastic activity producing new bone tissue
• inhibit the reabsorption of calcium in the kidneys

Question 28

T3 and T4 effect on BMR

Answer 28

boost the BMR of proteins, fats and carbohydrates

also vitamins

Question 29

what hormone do thyroid hormones work in conjunction with when controlling metabolism

Answer 29

growth hormone

Question 30

thyroid hormones as transcription factors

Answer 30

lipophilic and can cross the cell membrane and bind to intracellular receptors - which act as transcription factors etc

Question 31

Question 32

sick euthyroid syndrome

Answer 32

Non-thyroidal illness

Thyroid function tests in the acutely unwell patient are of limited value, and are only indicated when they are central to the clinical picture (e.g. profound hypothyroidism)

In a systemic illness, TFTs may become deranged. The typical pattern is for ‘everything to be low.’ The test should be repeated after recovery.

Question 33

where does lymph from the para/thyroid gland drain

Answer 33