2.2 Thyroid Hormones: Roles and Regulation

Question 1

Explain the production and release of thyroid hormones

Answer 1

• In thyroid follicular cells, iodine is concentrated from the blood
• It is oxidised into iodide and attached to tyrosine residues from thyroglobulin in the extracellular colloid
• As a result, T3 and T4 are formed, and released via the same capillaries where iodine was originally sourced from

Question 2

Describe how TSH levels vary based on time of day

Answer 2

• Lower in the morning until bed time
• Spike overnight and in the early hours of the morning

Question 3

Are thyroid hormones lipo or hydrophilic? How does this affect their transportation through the blood, and the kind of receptors they bind to?

Answer 3

• Thyroid hormones are lipophilic
• This means they travel through the blood bound to proteins (most commonly Thyroxine Binding Globulin (TBG))
• They also bind to intracellular receptors on target cells, since they are able to cross the phospholipid bilayer

Question 4

How are thyroid hormones metabolised?

Answer 4

Broken down in the liver (via sulfation)

Question 5

Describe the influence of thyroid hormones generally, and on skeletal muscle, bone, GI, and the cardiovascular system

Answer 5

• Generally: increased metabolism & thermogenesis (meaning increased oxygen consumption and energy usage)
• Muscle: Increased fast-twitch muscles
• Bone: Stimulates bone growth
• GI: Increased motility
• Cardiovascular; increased HR, and stroke volume (therefore higher cardiac output)