Thyroid hormones (dave's notes only so far)

Question 1

What does the thyroid gland secrete? From what cells?

Answer 1

Thyroid hormones from thyroid follicles, and calcitonin from C cells.

Question 2

What controls secretion of thyroid hormones?

Answer 2

Thyroid hormone secretion is under the control of TSH via TRH, with T4 acting on the pituitary and hypothalamus

Question 3

What increases thyroid hormone secretion?

Answer 3

Cold temperatures, presumably increases with increased TSH. “Higher centres have only a small influence”????

Question 4

What decreases thyroid hormone secretion?

Answer 4

Thyroid hormone secretion decreases with increased T4, decreases with somatostatin, stress, dopamine and glucocorticoids. “Higher centres have only a small influence”?????

Question 5

What are the principle thyroid hormones?

Answer 5

T3 and T4. A small amount of rT3. T3 is also formed in the peripheral tissues by conversion of T4 to T3.

Question 6

Which is more active? T3 or T4?

Answer 6

T3

Question 7

What happens to ingested iodine?

Answer 7

It is converted to iodide and absorbed

Question 8

What is the daily requirement of iodine? What happens to this?

Answer 8

Daily requirement is approximately 150ug/day. About 120ug enters the gland. Of this, ~80ug is made into hormone, and ~40ug diffuses into the ECF.

Question 9

How does iodide enter cells? Via what transport mechanism does this occur?

Answer 9

Via the Na+/I- symporter. This operates via a Na/K ATP-ase which then allows secondary active transport of I into the cell.

Question 10

What is the concentration gradient of Iodide between the ECF and follicular cells?

Answer 10

20-40x

Question 11

Describe thyroid hormone synthesis once iodide has entered follicular cells.

Answer 11

Peroxidase oxidises iodide back into iodine. Iodine then enters the colloid and binds to tyrosine residues which are attached to thyroglobulin. When secreted this colloid is “ingested????” providing free T4 and T3

Question 12

What can block the activity of peroxidase?

Answer 12

Thiourea, propylthiouracil.

Question 13

How much T4 and T3 are secreted each day?

Answer 13

80ug/day of T4 and 4ug/day of T3

Question 14

Is thyroid hormone secreted free or bound?

Answer 14

Secreted free, and it is free hormone which is active.

Question 15

What allows more uniform distribution of thyroid hormone?

Answer 15

Protein binding.

Question 16

Which proteins bind thyroid hormone?

Answer 16

Albumin, transthyretin and TBG. Albumin has the largest capacity to carry hormone, but TBG has the highest affinity.

Question 17

What fraction of T4 and T3 is bound by protein after secretion?

Answer 17

T4 - 99.98%

T3 - 99.8%

Question 18

Which of T4 and T3 has the shortest half life?

Answer 18

T3, and is the most active.

Question 19

What increases TBG?

Answer 19

Oestrogen and pregnancy

Question 20

What decreases TBG?

Answer 20

androgens “et al”

Question 21

How does foetal thyroid secretion differ to secretion after birth?

Answer 21

Much more rT3 is secreted, less T3. This switches after birth.

Question 22

What is the mechanism of action of thyroid hormone?

Answer 22

Thyroid hormone works via receptors on the nucleus (T3>T4) and ?? binds to DNA

Question 23

What is the calorigenic mechanism of thyroid hormone? What tissues are excluded?

Answer 23

Thyroid hormone increases O2 consumption in almost all metabolically active tissues, except brain, testes, uterus, anterior pituitary (?AP),

Question 24

What is the effect of T4 on the anterior pituitary?

Answer 24

T4 decreases O2 (oxidative??) activity in the anterior pituitary resulting in decreased TSH secretion.

Question 25

What is the effect the calorigenic mechanism of thyroid hormone due to?

Answer 25

The effect is due to an increase in fatty acid metabolism and an increase in Na/K+ ATP-ase activity. This results in increased energy requirements, vitamins et. al.

Question 26

What is the effect of thyroid hormone on the cardiovascular system and the central nervous system?

Answer 26

Increases cardiac output, increases electrical activity, ?irritability etc (?excitability maybe?)

Question 27

What is the effect of thyroid hormone on the musculoskeletal system and the gastrointestinal tract?

Answer 27

Muscle wasting, and in the GI tract increases absorption and fluctuations in carbohydrate metabolism (?CHO metal), increased N excretion.

Question 28

What may primary hypothyroidism be due to? What is the biochemical evidence of this?

Answer 28

May be due to failure of the thyroid gland. TSH will be high.

Question 29

What is secondary hypothyroidism due to? What is the biochemical finding consistent with this?

Answer 29

Pituitary or hypothalamic disease. TSH will be low.

Question 30

What are the clinical features of hypothyroidism?

Answer 30

General slowness, carotenemia, husky voice, hypercholesterolaemia, sparse dry hair, decreased reflexes.

Question 31

What is cretinism?

Answer 31

Hypothyroidism from birth resulting in decreased growth and mentation. May be from thyroid agenesis or genetic ??decrease in iodine?

Question 32

List six causes of hyperthyroidism

Answer 32

Grave’s disease, toxic multi nodular goitre, secondary to higher dysfunction (??), thyroiditis, ectopic tissue, exogenous hormone

Question 33

List seven clinical symptoms or signs of hyperthyroidism.

Answer 33

Nervousness, tremor, increased BMR, warm skin, increased heart rate, increased pulse pressure, AF

Question 34

Define pulse pressure!

Answer 34

The difference between the systolic and diastolic pressure - the pressure that causes the palpable pulse.