Endocrine Physiology - Thyroid

Question 1

Discuss the thyroid gland

Answer 1

Endocrine organ, weighs 25g in normal physiology
Anterior triangle of the neck bilaterally
Two lobes and one isthmus overlying the third tracheal ring
Occasional central pyramidal lobe

Produces thyroid hormones & calcitonin

Follicular cells secrete T3/T4
Modulates metabolic rate
Increase BMR, O2 consumption, carbohydrate absorption in GI tract, lipolysis, production & sensitisation of β adrenoreceptors

Parafollicular (C-cells) secrete calcitonin
Calcium homeostasis (decreases plasma calcium concentration)

Blood supply:
Superior thyroid from external carotid
Inferior thyroid from subclavian
Superior, middle, and inferior thyroid veins.

Innervation:
Recurrent & external laryngeal nerves

Question 2

What is the thyroid axis, and how is thyroid hormone produced?

Answer 2

Hypothalamus -> TRH ->
Pituitary - > TSH ->
Thyroid -> T4/T3 (T4 also converted peripherally)

T4&T3 inhibit hypothalamus and pituitary

Iodine enters the follicle, where thyroid peroxidase joins it to tyrosine to form MIT (mono-iodotyrosine) and DIT (Di-iodotyrosine).

These are joined together with a thyroglobulin protein, and coupled with another MIT/DIT

T3 = 1x MIT, 1x DIT
T4 = 2x DIT

This is stored in a lysosome, ready for release into the bloodsteam when cleaved from thyroglobulin.

Question 3

At what receptors do thyroid hormones act?

Answer 3

T3 triggers nuclear receptors, and thus changes in mRNA expression

T4 mostly converted to T3, but some activity at thyroid receptor

30-40% converted to T3
40% converted to reverse T3

rT3 is a competitive agonist at the receptor - possibly why some patients are clinically hypothyroid when T3/T4 in normal range.