L7 - Thyroid hormone

Question 1

Describe the structure of the thyroid gland?

Answer 1

Comprised of follicles each consisting of a monolayer of epithelial cells enclosing a large core of viscous, homogenous colloid.
Colloids act as a reservoir of thyroid hormone

Question 2

What does the thyroid gland secrete ?

Answer 2

Triodothyronine (T3)
Tetraiodothyronine (T4 - Thyroxine)
Calcitonin - calcium homeostasis

Question 3

Production of T3/4

Answer 3

Iodine + tyrosine = MIT and DIT
MIT + DIT = T3
DIT and DIT = T4
in tissues T4 is converted to T3 by iodothyronine deioidinase
MIT and DIT are rapidly degraded by halogenases to free the iodide which is reutilised by combination with thyroglobulin
t3 and t4 leave follicular cells and enter blood for distribution to target tissues
95% of thyroid hormone leaving the gland is in form of T4. within target tissue T4 Converts to T3 (80%) or reverse T3 (20%)
T3 has biological activity approx 40x more than T4
reverse T3 bio inactive

Question 4

half lives

Answer 4

T4 - 6/8 Days

T3 - 1 day

Question 5

How is thyroid hormone transported around the body?

Answer 5

transported in blood bound to plasma proteins as not water soluble.
75% of T4 binding is to TBG
15-20% TBPA (thyroxine binding prealbumin)
5-10% albumin

Question 6

Thyroid receptor

Answer 6

intracellular - nuclear

influence gene transcription thus protein synthesis

Question 7

Thyroid hormone mechanism

Answer 7

increase basal metabolic rate
increase carbohydrate metabolism
increase synthesis and mobilisation of lipids
essential for normal development of CNS esp myelination of nerves

Primary mechanism: increase in number and size of mitochondria

Question 8

Levothyroxine

Answer 8

treat thyroid deficiency
Can be used to suppress TSH secretion in treatment of some thyroid tumours.
Adverse effects - excessive doses cause palpitations, arrhythmias, diarrhoea, insomia, tremor, weight loss

Question 9

Anti-thyroid drugs

Answer 9

Carbimazole and methimazole -
inhibit synthesis of thyroid hormones by preventing incorporation of iodide into the thyroglobulin

PTU - prevents conversion of T4-T3

Potassium perchlorate - competes with iodide for the active iodide uptake pump

Question 10

Adverse effects of Carbimazole

Answer 10

Rashes, pruritis, more seriously - neutropenia, agranulocytosis

Question 11

Jod-Basedow phenomenon

Answer 11

Prophylactic administration of iodine either by injection or by incorporation into table salt or flour has markedly reduced the incidence of endemic goitre worldwide, although it carries with it the risk of Jod-Basedow phenomenon in which iodine administration precipitates hyperthyroidism.

Question 12

Drugs which can induce goitre:

Answer 12

Drugs which can induce goitre:
lithium which is used in the treatment of bipolar depression,
iodides which are contained in vitamin preparations and some cough remedies
These ions are selectively concentrated within the thyroid gland where they interfere with iodide incorporation and hormone release.