Lecture 5: The Thyroid Gland

Question 2

name the two physiologically active forms of thyroid hormones

Answer 2

T3 (triiodothyronine) and T4 (thyroxine)

Question 3

what are the two cells types of present within the thyroid gland and what is their function?

Answer 3

1. C (clear) cells which secrete calcitonin (Ca2+ regulating hormone).
2. follicular cells which support thyroid hormone synthesis and surround hollow follicles.

Question 4

describe thyroid follicles

Answer 4

• spherical structures whose walls are made of follicular cells.
• centre of follicle filled with colloid = sticky glycoprotein marix > contains 2-3 months supply of TH.

Question 5

function of follicular cells

Answer 5

• manufacture the enzymes that make thyroid hormones as well as thyroglobulin, a large protein rich in tyrosine residues.
• also actively concentrate iodide from the plasma and transport it to colloid where it combines with the tyrosine residues to form the thyroid hormones.
• both tyrosine and iodide are derived from the diet.

Question 6

how is iodide transported through the thyroid gland?

Answer 6

• iodide enters the follicular cells from the plasma via Na+/I- transporter (symport). The coupling to Na+ enables the follicular cells to take up iodide against a conc. gradient.
• iodide is then transported into the colloid via pendrin transporter.

Question 7

action of thyroid peroxidase (aka thyroperoxidase)?

Answer 7

• catalyses the oxidation of iodide to iodine and the addition of iodine to tyrosine residues on the thyroglobulin molecule to create thyroid hormone.

Question 8

addition of one iodine to tyrosine creates

Answer 8

MIT (monoiodotyrosine)

Question 9

adding a second iodine to tyrosine resides on thyroglobulin molecule creates

Answer 9

DIT (diiodotyrosine)

Question 10

conjugation of MIT + DIT =

Answer 10

T3 (triiodothyronine)

Question 11

conjugation of DIT + DIT =

Answer 11

T4 (thyroxine)

Question 12

what does TSH activate?

Answer 12

• in response to TSH, portions of the colloid are taken back up within the follicular cell by endocytosis.
• within the cells they are packages into vesicles containing proteolytic enzymes that cut the thyroglobulin to release thyroid hormones.

Question 13

are T3 and T4 lipid or water soluble?

Answer 13

lipid soluble

Question 14

how do T3 and T4 travel through the blood?

Answer 14

bound to plasma proteins, mainly thyroxine-binding globulin

Question 15

does TBG have a higher affinity for T3 or T4?

Answer 15

T4, partly accounts for longer half-life

Question 16

what % of plasma thyroid hormone is unbound (free)?

Answer 16

0.2%
- ONLY free hormone exerts an inhibitor effect on TSH and TRH and is physiologically active.

Question 17

most TH circulates in the form of protein bound T3 or T4?

Answer 17

T4

Question 18

90% of TH binding to TH receptors inside cells is T3 or T4?

Answer 18

T3 > TH receptor has a much higher affinity than T4 making it 3-5 times more physiologically active than T4.

Question 19

which is more physiologically active: T3 or T4?

Answer 19

T3 >TH receptor has a much higher affinity for T3 than T4 making it 3-5 times more physiologically active than T4.

Question 20

what factors stimulate increased TRH secretion from hypothalamus?

there is always a tonic release

Answer 20

cold
exercise
pregnancy

Question 21

how are glucocorticoids and somatostatin inhibitory to thyroid hormone release

Answer 21

• SS inhibits TSH (TH required for HG action)
• GC inhibits TSH and conversion of T4 to T3.

Question 22

thyroid hormone function

Answer 22

• raises metabolic rate and promotes thermogenesis, typically through promoting futile cycles of simultaneous catabolism and anabolism.
• increases hepatic gluconeogenesis
• net increase in proteolysis
• net increase in lipolysis
• critical for growth (stimulates GH receptor expression)
• essential for brain development in utero

Question 23

describe Grave’s disease

Answer 23

• a common cause of hyperthyroidism.
• antibodies produces that bind and mimic TSH > continuous activation of 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.

Question 24

describe a thyroid adenoma

Answer 24

• a rare cause of hyperthyroidism
• hormone-secreting thyroid tumour

Question 25

symptoms of hyperthyroidism

Answer 25

1. 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 e.g. anxiety
4. elevated CV function (TH is permissive to epinephrine, beta receptors) > increased HR/contractile force, high output, cardiac failure

Question 26

causes of hypothyroidism

Answer 26

• Hashimoto’s disease - autoimmune attack of thyroid gland
• deficiency in dietary iodine
• idiopathic

Question 27

symptoms of hypothyroidism

Answer 27

1. 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 CV function > slow heart rate/weaker pulse

Question 28

describe goitre

Answer 28

• both hypo- and hyperthyroidism can result in hypertrophy of thyroid gland, termed goitre.
• 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)