E7 - Thyroid and adrenal glands

Question 1

Where is the thyroid?

Answer 1

• Sits under the Adam’s apple, “where the bowtie would”

- Half a plum in size

Question 2

What hormones are secreted from the thyroid gland and from which parts?

Answer 2

• Thyroid hormones; thyroxine (T4) and tri-iodothyronine (T3) - synthesised and stored in the colloid (highly protein rich viscous bit) in the follicle
• Calcitonin; synthethised in parafollicular ‘C’ cells (situated between follicles)
• Parathyroid hormone; secreted by parathyroid glands (4 small glands on the posterior surface of the thyroid gland)

Question 3

What do the hormones calcitonin and parathyroid hormone do?

Answer 3

• Calcitonin; act to reduce plasma Ca2+ levels

- PTH: increases plasma Ca2+ levels (opposing effects)

Question 4

What do the numbers after T3 and T4 respectively denote?

Answer 4

The number of iodine residues attached:
T3: tri-iodothyronine (MIT + DIT); 5-10%
T4: thyroxine (DIT + DIT); 90-95%

Question 5

Does the iodine coupling DIT + MIT have biological activity?

Answer 5

= rT3; not biologically active but acts as antagonist at T3 receptors.

Question 6

Outline the endocrine axis for thyroid hormone secretion

Answer 6

• Hypothalamus
  > Releases TRH (thyrotropin-releasing hormone)
• Anterior pituitary
  > Releases TSH (thyroid-stimulating hormone; thyrotropin)
• Thyroid gland
  > Secretes T3/T4 (-ve feedback to above)
• Target cell response

Question 7

If 99.9% of T4 and 99% of T3 are bound to plasma proteins (they are lipophilic) in the blood, how do they end up carrying out an effect?

Answer 7

• Deiodinase enzymes at the target cell/tissue (liver, kidney) are able to release the free hormone
• T4 (thyroxine, 90-95% of thyroid hormone release) is converted into T3 (or rT3 too), which is 4-5X more potent than T4 (which still has activity)

Question 8

How is the hormone [T4] measured to determine thyroid function and why?

Answer 8

• Free T4 correlates more closely to thyroid function than Total T4 due to the large amount unavailable for biological action
• T4 functions as a prohormone, T3 as the principal active hormone

Question 9

What are the actions of thyroid hormones?

Answer 9

• Increase metabolism in the body; generalised increase in functional activity of almost all tissues (BMR is increased, as is metabolism of CHOs/proteins/fats all the way to heat and not for energy)
• Stimulates growth and development (important in new-born, important for normal development and CNS maturation)
• Synergistic with actions of SNS/catecholamines (thyroid hormones upregulate β-adrenoceptors in heart and vasculature)