Thyroid And Parathyroid (Physiology)

Question 1

List the principal thyroid hormones

Answer 1

• Triiodothyronine (T3)

- Thyroxine also called tetraiodothyronine (T4)

Question 2

Describe the hypothalamic-pituitary-thyroid axis and its role in control of thyroid gland function

Answer 2

• A stimulus e.g. cold environment, results in the hypothalamus secreting thyrotropin-releasing hormone (TRH).
• TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH).
• TSH stimulates the thyroid gland to secrete thyroid hormone (TH).
• TH stimulates an increase in basal metabolic rate, protein synthesis, and sympathetic tone.
• Once the level of TH is high enough, TH inhibits the release of TSH by the anterior pituitary. This is known as negative feedback inhibition.
• To a lesser extent, TH also inhibits the release of TRH by the hypothalamus.
• Stress can decrease the amount of thyroid secreted by inhibiting the release of TRH by the hypothalamus.

Question 3

Understand the principal effects of thyroid hormones in growth and development, and in the healthy adult

Answer 3

• The primary effect of thyroid hormones is to increase one’s metabolic rate, and as a result, they raise oxygen consumption and have a calorigenic effect (increases heat production).
• To ensure an adequate blood supply to meet this increased demand, they raise the respiratory rate, heart rate, and strength of the heartbeat.
• They stimulate the appetite and accelerate the catabolism of carbohydrates, fats, and protein for energy.
• Promote alertness and quicker reflexes; growth hormone secretion; growth of the bones, skin, hair, nails, and teeth; and development of the foetal nervous system.
• Also promote normal muscular development, and function; normal GI motility and tone; and normal female reproductive ability and lactation.

Question 4

What are the consequences for the body of a deficiency in secretion of the thyroid hormones

Answer 4

• BMR below normal; decreased body temperature, cold intolerance, decreased appetite, and weight gain.
• Decreased glucose metabolism, elevated blood cholesterol levels, and decreased protein synthesis.
• Slow brain development in infants, while adults experience depression, paresthesia, and hypoactive reflexes.
• Decrease in strength of heartbeat, low heart rate, and low blood pressure.
• Growth retardation in children and joint pain in adults.
• Depressed GI motility and tone.
• Depression of ovarian function and lactation.

Question 5

What are the consequences for the body of excess secretion of the thyroid hormones

Answer 5

• BMR above normal, increased body temperature, heat intolerance, increased appetite,and weight loss.
• Enhanced catabolism of glucose, protein, and fats; weight loss; and loss of muscle mass.
• Irritability, insomnia, and exophthalmos (in Graves’ disease).
• Rapid heart rate and possible palpitations, high blood pressure, and if prolonged can cause heart failure.
• Muscle atrophy and weakness.
• Excessive skeletal growth initially, followed by early epiphyseal closure and short stature; demineralisation of skeleton in adults.
• Excessive GI motility and diarrhoea.
• Depressed ovarian function.

Question 6

Describe the role of calcium in the body

Answer 6

• Strong bones and teeth
• Muscle contraction
• Co-factor in metabolic reactions
• Cell division
• Plasma membrane integrity
• Neuronal excitability
• Glycogen metabolism

Question 7

List the functional calcium pools in the body

Answer 7

• A stable pool of calcium (in bones) which is incorporated into hydroxyapatite and is not easily exchanged with the blood.
• Exchangeable calcium (in intestine) which is 1% or less of the total but is easily released to the tissue fluid.

Question 8

Outline the biological significance of free calcium in the extracellular fluid

Answer 8

• Calcium travels from bone by resorption to act as a buffer in the extracellular fluid.
• Free calcium in plasma enters cells and reacts with proteins in cells to cause a physiological response e.g. causes neuronal excitability.

Question 9

Name the hormones involved in calcium regulation, identify their sources, and list their main actions

Answer 9

Parathyroid hormone (parathormone)

• Parathyroid glands.
• Indirectly activates osteoclasts which resorb bone and raise blood calcium concentration; inhibits urinary calcium excretion; and promotes calcitriol synthesis.

Activated vitamin D (1,25-dihydroxycholecalciferol or calcitriol)

• Sunlight.
• Promotes intestinal absorption of calcium; reduces urinary excretion of calcium; promotes both resorption and mineralisation; and stimulates osteoclast activity.

Calcitonin

• Secreted by parafollicular cells in the thyroid gland.
• Antagonises parathyroid hormone and stimulates osteoblast activity, thus promoting calcium deposition and bone formation. Inhibits bone resorption and increases calcium excretion by the kidneys.

Question 10

What are the causes of hypothyroidism

Answer 10

• Thyroiditis e.g. Hashimoto’s disease
• Severe iodide deficiency
• Severe deficiency of one or more synthesis enzymes
• Removal or destruction of thyroid gland

Question 11

What are the causes of hyperthyroidism or thyrotoxicosis

Answer 11

• Autoimmune disease - Graves’ disease (most common cause)
• Thyroid adenoma
• Inappropriate thyroid-stimulating hormone secretion (rare)

Question 12

What causes Graves’ disease

Answer 12

• Cells produce antibodies which bind to TSH receptors.

- This causes excessive production of the thyroid hormones.

Question 13

List the causes of rickets and osteomalacia

Answer 13

• Lack of dietary vitamin D and/or sunlight.
• Malabsorption of fats.
• Failure to form calcitriol.