Thyroid Gland - Anatomy & Physio

Question 1

Describe structure of the thyroid gland.

Answer 1

• butterfly-shaped gland in the anterior neck
• two lobes connected by an isthmus
• made up of follicles, and parafollicular cells in between follicles

Question 2

What are the main functions of thyroid gland?

Answer 2

1. Produces thyroid hormones (T3, T4)
   
   • these hormones regulate metabolism, growth, development
2. Produces calcitonin
   
   • calcitonin lowers blood calcium

Question 3

Synthesis of thyroid hormones

Answer 3

1. Thyroglobulin synthesised in follicular cells, & secreted into colloid

2. I⁻ ion (in bloodstream) enters follicular cell from basolateral side via Na⁺/I⁻ symporter and go into colloid

3. In colloid, I⁻ oxidised into I₂

4. I₂ iodinates tyrosyl residues on Thyroglobulin - forming MIT & DIT

5. MIT + DIT → T3 hormone
DIT + DIT → T4

Question 4

Where are thyroid hormones stored?

Answer 4

Within colloid in lumen of thyroid follicles.

• bound to iodinated thyroglobulin

Question 5

Secretion of thyroid hormone

Answer 5

1. Thyroglobulin in colloid is endocytosed into follicular cell

2. Lysosome inside follicular cell fuses with vesicle

3. Lysosome digests thyroglobulin, releasing free T3 & T4 into cytoplasm

• 90% T4 , 10% T3

4. T3 & T4 released into bloodstream

Note: uncoupled MIT & DIT are deiodinated & iodine is recycled

Question 6

Regulation of thyroid hormones secretion

Answer 6

Negative feedback mechanism

• When T3, T4 levels ↓, Hypothalamus releases thyrotropin-releasing hormone TRH

(TRH stimulates anterior pituitary gland to release thyroid-stimulating hormone TSH)

• TSH stimulates thyroid gland to produce & release thyroid hormones

Question 7

T3 v/s T4

Answer 7

T3:
- 3 iodine atoms
- more biologically active
- synthesised from T4
- shorter half-life (1 day)

T4:
- 4 iodine atoms
- less active
- directly synthesised
- longer half-life (7 days)

Question 8

Transport of thyroid hormones in bloodstream

Answer 8

• Once in circulation, 70% thyroid hormones travel bound to thyroxine-binding protein TBP
• 10 - 15% bound to transthyretin
• small fraction travels unbound / free

Question 9

Effects of thyroid hormones

Answer 9

on Heart
→ increase affinity of beta-adrenergic receptors
→ increase ventricular contractility, hence increasing cardiac output

on Adipose tissue
→ stimulate lipolysis

on Muscles
→ stimulate protein breakdown

on Bones + Nervous system
→ promote normal development

on Gut
→ increase rate of carbohydrate absorption

Question 10

Cardiovascular effects of thyroid hormone

Answer 10

Indirect Effects:
↑heat & CO₂, leading to↓vascular resistance

↓diastolic BP, leading to↑adrenergic tone

Direct Effects:
↑beta-adrenergic signalling
↑Na⁺/K⁺ ATPase
↑Contractility and Cardiac output

All lead to ↑heart rate, cardiac output, blood volume

Question 11

Where is T4 activated?

Answer 11

In target cells

T4 → T3

Question 12

What does TSH do?

Answer 12

• increase thyroglobulin synthesis
• increase iodine uptake
• increase iodination
• increase endocytosis of colloid
• increase lysosomal activity
• increase follicular cell size