Thyroid gland (3)

Question 1

Where are the thyroid glands situated?

Answer 1

The two lobes of the butterfly-shaped gland are found on either side of the trachea just below the larynx (voice box) in the front of the neck.

Question 2

What hormone does the thyroid secrete?

Answer 2

The most important hormone secreted by the thyroid is thyroxine.

Question 3

What is essential for the secretion of thyroxine? (2)

Answer 3

• Note: Iodine is an essential element needed for the production of thyroxine. Iodine is found in sea food, sea salt or iodized salt.
• A goitre may develop if levels of iodine are low.

Question 4

What are the functions of thyroxine? (3)

Answer 4

1. It increases the basal metabolic rate (and therefore production of body heat) in all body cells. It does this by controlling the rate of cellular respiration.
2. It promotes the normal functioning of the heart.
3. It promotes the normal functioning of the nervous system by increasing nervous activity and sharpening alertness and reflexes.

Question 5

What is basal metabolic rate?

Answer 5

Basal metabolic rate = amount of energy the body needs to keep functioning while at rest.

Question 6

What is Hypothyroidism?

Answer 6

Producing too little thyroxine, causes a low metabolic rate.

Question 7

What does Hypothyroidism result in for adults?

Answer 7

In adults hypothyroidism results in, myxoedema, a condition of mental and physical sluggishness, with low blood pressure, a slow heart and respiratory rate and also a low body temperature.

Question 8

What does Hypothyroidism result in?

Answer 8

In children hypothyroidism results cretinism, a condition in which a child does not grow physically, has immature sexual development and is mentally retarded.

Question 9

How can Hypothyroidism be treated?

Answer 9

Hypothyroidism can be treated with iodine supplements or synthetic iodine. However, once developmental and mental abnormalities have occurred in a child, they cannot be reversed.

Question 10

What is Hyperthyroidism?

Answer 10

Hyperthyroidism, i.e. producing too much thyroxine, causes a high metabolic rate.

Question 11

What are the effects of hyperthyroidism? (4)

Answer 11

• The body temperature is high and sweating increases.
• The heart and respiratory rate and blood pressure increases.
• There are muscular tremors and nervousness.
• Sufferers often have a swollen thyroid gland, known as an exophthalmic goitre.

Question 12

How can the amount of thyroxin in the blood be kept constant?

Answer 12

This is done by a control mechanism known as negative feedback.

Question 13

How do thyroid and pituitary glands interact to keep the amount of thyroxin in the blood constant?

Answer 13

• The pituitary detects a decreased level of thyroxine in the blood, so it secretes more TSH.
• The TSH stimulates the thyroid to secrete more thyroxine, returning its level to normal.
• The higher level of thyroxine inhibits further secretion of TSH from the pituitary.

The negative feedback mechanism will therefore ensure that the level of thyroxine in the blood is kept at the correct level (set point) at all times.