Thyroid hormones (dave's notes only so far) Flashcards
What does the thyroid gland secrete? From what cells?
Thyroid hormones from thyroid follicles, and calcitonin from C cells.
What controls secretion of thyroid hormones?
Thyroid hormone secretion is under the control of TSH via TRH, with T4 acting on the pituitary and hypothalamus
What increases thyroid hormone secretion?
Cold temperatures, presumably increases with increased TSH. “Higher centres have only a small influence”????
What decreases thyroid hormone secretion?
Thyroid hormone secretion decreases with increased T4, decreases with somatostatin, stress, dopamine and glucocorticoids. “Higher centres have only a small influence”?????
What are the principle thyroid hormones?
T3 and T4. A small amount of rT3. T3 is also formed in the peripheral tissues by conversion of T4 to T3.
Which is more active? T3 or T4?
T3
What happens to ingested iodine?
It is converted to iodide and absorbed
What is the daily requirement of iodine? What happens to this?
Daily requirement is approximately 150ug/day. About 120ug enters the gland. Of this, ~80ug is made into hormone, and ~40ug diffuses into the ECF.
How does iodide enter cells? Via what transport mechanism does this occur?
Via the Na+/I- symporter. This operates via a Na/K ATP-ase which then allows secondary active transport of I into the cell.
What is the concentration gradient of Iodide between the ECF and follicular cells?
20-40x
Describe thyroid hormone synthesis once iodide has entered follicular cells.
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
What can block the activity of peroxidase?
Thiourea, propylthiouracil.
How much T4 and T3 are secreted each day?
80ug/day of T4 and 4ug/day of T3
Is thyroid hormone secreted free or bound?
Secreted free, and it is free hormone which is active.
What allows more uniform distribution of thyroid hormone?
Protein binding.
Which proteins bind thyroid hormone?
Albumin, transthyretin and TBG. Albumin has the largest capacity to carry hormone, but TBG has the highest affinity.
What fraction of T4 and T3 is bound by protein after secretion?
T4 - 99.98%
T3 - 99.8%
Which of T4 and T3 has the shortest half life?
T3, and is the most active.
What increases TBG?
Oestrogen and pregnancy
What decreases TBG?
androgens “et al”
How does foetal thyroid secretion differ to secretion after birth?
Much more rT3 is secreted, less T3. This switches after birth.
What is the mechanism of action of thyroid hormone?
Thyroid hormone works via receptors on the nucleus (T3>T4) and ?? binds to DNA
What is the calorigenic mechanism of thyroid hormone? What tissues are excluded?
Thyroid hormone increases O2 consumption in almost all metabolically active tissues, except brain, testes, uterus, anterior pituitary (?AP),
What is the effect of T4 on the anterior pituitary?
T4 decreases O2 (oxidative??) activity in the anterior pituitary resulting in decreased TSH secretion.
What is the effect the calorigenic mechanism of thyroid hormone due to?
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.
What is the effect of thyroid hormone on the cardiovascular system and the central nervous system?
Increases cardiac output, increases electrical activity, ?irritability etc (?excitability maybe?)
What is the effect of thyroid hormone on the musculoskeletal system and the gastrointestinal tract?
Muscle wasting, and in the GI tract increases absorption and fluctuations in carbohydrate metabolism (?CHO metal), increased N excretion.
What may primary hypothyroidism be due to? What is the biochemical evidence of this?
May be due to failure of the thyroid gland. TSH will be high.
What is secondary hypothyroidism due to? What is the biochemical finding consistent with this?
Pituitary or hypothalamic disease. TSH will be low.
What are the clinical features of hypothyroidism?
General slowness, carotenemia, husky voice, hypercholesterolaemia, sparse dry hair, decreased reflexes.
What is cretinism?
Hypothyroidism from birth resulting in decreased growth and mentation. May be from thyroid agenesis or genetic ??decrease in iodine?
List six causes of hyperthyroidism
Grave’s disease, toxic multi nodular goitre, secondary to higher dysfunction (??), thyroiditis, ectopic tissue, exogenous hormone
List seven clinical symptoms or signs of hyperthyroidism.
Nervousness, tremor, increased BMR, warm skin, increased heart rate, increased pulse pressure, AF
Define pulse pressure!
The difference between the systolic and diastolic pressure - the pressure that causes the palpable pulse.
