Thyroid Physiology

Question 1

What is the name and type of pump that brings iodide into cells?

Answer 1

Na/I symport pump - pendrin

Question 2

What protects against short term iodine deficiency?

Answer 2

Thyroperoxidase creates mature hormones from MIT and DIT.

The MIT and DIT remain connected to TGB storage molecule until needed - allows for storage of TH precusor in case of short term iodine deficiency

Question 3

What transporter releases T3 and T4 from cells into circulation?

Answer 3

Monocarboxylate transporter 8 (MCT8)

Question 4

How do thyroid hormones enter cells?

Answer 4

Through iodothyronine transporters:

• Organic anion transporters (OATPs, passive)
• Monocarboxylate transporter (MCT8/10, active)

Question 5

How do T4 and T3 circulate in blood?

Answer 5

Mostly bound:

• TBG 70%
• Trasnthyretin
• Albumin

Unbound:

• T4 - 0.03%
• T3 - 0.3%

Question 6

Why is 90% of thyroid hormone release T4?

Answer 6

T4 has a longer half life (6days) compared to T3 (3 days)

Target cell deiodinase enzymes convert T4 to T3

• TH receptors have a much higher affinity for T3 to cause effect.

Question 7

Describe the H-P-T axis, including steps of release, stimuli for release, stimuli for inhibition and negative feedback controls

Answer 7

Cold stimulates release

Hypothalamus → Thyrotropin releasing hormone (TRH) → AP → TSH → Thyroid gland → T3, T4

T3 and T4 produce negative feedback on the AP and hypothalamus.

Question 8

What does TSH do?

Answer 8

Binds TSH-r, increase cAMP

1. INcrease hormone synthesis
2. Increase hormone secretion
3. Increase blood flow to thyroid.

Question 9

What is thyroid hormone the main determinant of?

Answer 9

THe basal metabolic rate

Question 10

What are the 4 categories of metabolism the thyroid hormone is responsible for regulating, and how does it affect them?

Answer 10

Oxidative metabolism

• ATP breakdown and energy utilisation

CHO metabolism

• CHO utilisation
• Increased glucose absorption by GIT, increased glucose oxidation in liver/fat/muscle

Lipid Metabolism

• Mobilisation of fat from fat cells
• Increases lipolysis and lipogenesis

Nitrogen Metabolism

• Protein synthesis and degradation

Question 11

What are the 4 categories of metabolism the thyroid hormone is responsible for regulating, and how does it affect them?

Answer 11

Oxidative metabolism

• ATP breakdown and energy utilisation

CHO metabolism

• CHO utilisation
• Increased glucose absorption by GIT, increased glucose oxidation in liver/fat/muscle

Lipid Metabolism

• Mobilisation of fat from fat cells
• Increases lipolysis and lipogenesis

Nitrogen Metabolism

• Protein synthesis and degradation

Question 12

What systemic effects do thyroid hormones influence?

Answer 12

Temperature

• Energy utilisation produces heat
• Brown adipose tissue thermogenesis

CV REgulation

• Increased tissue blood flow
• Increased cardiac output by increasing SNS activity

Skeletal system

• Bone formation and maturation

CNS

• Brain development