Lecture 27 - Thyroid gland, parathyroid gland and adrenal glands

Question 1

Thyroid gland

Answer 1

Thyroid gland secrete hormones that affect metabolic rate and calcium ion levels in body fluids

Located just below the larynx (located low in front of the throat). It is on the anterior and lateral surfaces of the trachea (butterfly shape, the back is open)

Question 2

Hormones made and secreted from the thyroid gland

Answer 2

Thyroid hormone - essential for optimal metabolic activity

Calcitonin - hormone involved in calcium homeostasis

Question 3

Structure of the thyroid gland

Answer 3

Composed of small spherical sacs called follicles

Each follicle is surrounded by follicular cells (simple cuboidal epithelial cells) and is the site of thyroid hormone synthesis.

Clear cells or C cells lie in clusters between the follicles and make the hormone calcitonin.

Question 4

Follicular cells

Answer 4

Cell in the thyroid gland

Each follicle is surrounded by follicular cells (simple cuboidal epithelial cells) and is the site of thyroid hormone synthesis.

Question 5

Clear cells (c cells)

Answer 5

Cell in the thyroid gland

Clear cells or C cells lie in clusters between the follicles and make the hormone calcitonin.

Question 6

Thyroid hormone synthesis

Answer 6

Iodine enters follicle cell from blood and travels into follicle cavity

Follicle cells release protein, thyroglobulin (TGB) into follicle

Iodine reacts with tyrosine in the TGB molecules in the follicle cavity

Iodised TGB moves into the follicular cells

Thyroid hormones detach from TGB as needed (T3 is the active form (3 iodines bound) and T4 is the more plentiful form (4 iodines bound))

T3 and T4 travel bound to a carrier protein (thyroid-binding globulin, TBG) to target cells through the blood

Question 7

Why is iodine so important?

Answer 7

Iodine is a necessary part of our diet as it is required for the production of thyroid hormone

Question 8

Target cell activation by thyroid hormone

Answer 8

Made in advance and stored until required

Travels bound to a carrier protein

Detaches from the carrier protein and enters the target cell

T3 binds to the T3 receptor in the nucleus (receptor is already bound to the specific DNA site- so that as soon as the thyroid hormone binds, you can get an up regulation of that gene. These specific genes are activated to transcribe mRNA. mRNA translation occurs to get the formation of specific proteins)

Specific genes are activated to transcribe messenger RNA (mRNA)

mRNA translation occurs in the cytoplasm and specific proteins are synthesised e.g. the sodium potassium pump

Response time is 45 minutes to days

Note that this is different to other lipid soluble hormones i.e. different to the steroid hormones

Question 9

Control of thyroid hormone secretion

Answer 9

External and internal stimuli cause the hypothalamus to secrete thyrotropin-releasing hormone/TRH, which leads the anterior pituitary gland to secrete thyroid-stimulating hormone/TSH and thyroid gland secretes thyroid hormones (T3 and T4)

T3 and T4 negatively feedback to the hypothalamus (preventing TRH release) and the anterior pituitary (TSH)

Question 10

Effects of thyroid hormone

Answer 10

Increase in basal metabolic rate by the increase in synthesis and activity of the sodium and potassium pump
Stimulates growth (foetus and early childhood)
Nervous system - normal alertness and reflexes

Question 11

Thyrotropin-releasing hormone

Answer 11

produced by neurons in the hypothalamus, that stimulates the release of thyroid-stimulating hormone (TSH) and prolactin from the anterior pituitary.

Question 12

Thyroid-stimulating hormone

Answer 12

Thyroid stimulating hormone is produced by the pituitary gland. Its role is to regulate the production of hormones by the thyroid gland.

Question 13

Thyroid hormones (T3, T4)

Answer 13

The thyroid produces a hormone called T3 and it also produces a hormone called T4. Together, these hormones regulate your body’s temperature, metabolism, and heart rate.

Thyroid cells are the only cells in the body which can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism

Question 14

What is the basal metabolic rate? (BMR)

Answer 14

BMR is the rate of energy expenditure under basal conditions (basal conditions = minimal activity level that is necessary for life)
Awake, resting, lying down, have fasted and in comfortable temperature
BMR declines with age
Diet intake lowers with age due to the energy required to meet BMR
Males generally have a higher BMR (and a higher % of muscle tissue) - muscle at rest uses more energy

Question 15

Actions of thyroid hormone

Answer 15

Required for normal growth, alertness and metabolism

Question 16

Thyroid hormone effects on metabolism

Answer 16

Increased body heat production (increases oxygen consumption and ATP hydrolysis)

Stimulates fatty acid oxidation (usage of stored fats) in many tissues

Increases proteolysis, predominantly from muscle

Stimulates carbohydrate metabolism (usage of stored glucose), enhances insulin dependent entry of glucose into cells, increases gluconeogenesis and glycogenolysis

Note that is does not cause significant increase in blood glucose (blancaced increases in production and uptake into/usage by cells - they effectively cancel each other out)

Question 17

Where does our calcium come from?

Answer 17

1000 mg/day is the dietary requirement

Faecal loss = 350mg/day
Urinary loss = 650 mg/day
Plasma levels = 8.5-11 mg/dL

Absorbed in the gut and lost in the faeces
Filtration and reabsorption by kindey
Resorption from bones and deposition by bone
These fluctuations are not ideal for daily needs therefore homeostatic balance is needed for blood calcium

Question 18

Calcium homeostasis

Answer 18

Calcium is essential for many physiological functions such as contraction of muscles and release of neurotransmitters

The concentration of extracellular (blood) calcium is tightly controlled.

Bone is the body’s major store of calcium. Immediate adjustments can be made by rapid exchanges of calcium between bone and the blood

Normally a balance exists between the constant build-up and breakdown of bone

Three hormones are involved in calcium regulation - parathyroid hormone (PTH), calcitriol and calcitonin

Question 19

Parathyroid hormone (PTH)

Answer 19

Hormone secreted by the parathyroid glands

Question 20

Calcitonin

Answer 20

hormone involved in calcium homeostasis

made and released at the thyroid gland

Question 21

Parathyroid glands

Answer 21

Parathyroid gland secretes a hormone important to the regulator of calcium ion levels in body fluids (PTH)

Located on the posterior aspect of the thyroid gland are 4 bean looking structures which are known as the parathyroid glands

Question 22

Control system when there is low blood calcium

Answer 22

Release regulated via negative feedback

Calcitriol (active form of vitamin D) is produced by the kidney in response to PTH and increases absorption of Ca2+ from the intestines (note that the calcitriol has a negative feedback system with itself)

PTH also increases Ca2+ reabsorption at the kidneys and decreases urinary excretion

PTH increases bone breakdown (increasing the release of Ca2+ into the bloodstream)

These effects mean that blood Ca2+ rises and this negatively feedback to the parathyroid glands to stop producing PTH

Question 23

Calcium regulation - PTH and calcitonin

Answer 23

Decrease in plasma calcium leads to increase in PTH release (when calcium is less than 8.5 mg/dL)
Increase in plasma calcium leads to decrease in PTH release (when calcium is greater than 11 mg/dL)

This pathway is not controlled by the hypothalamus. The parathyroid glands are the control centre and sensor for this system and the effects or bone, intestine and kidneys

Question 24

Control system when there is high blood calcium

Answer 24

Decrease in PTH leads to increase in osteoblastic activity (formation of bone, increase in calcium deposition into bone). And there is an increase in calcium lost from the body/excreted in the urine. And vitamin D conversion to calcitriol decreases so decrease calcium absorption from the digestive tract. Overall this will decrease blood calcium concentration and restore it to set point which is negative feedback as you are reducing the stimulus until it is removed completely.

Question 25

Adrenal glands structure

Answer 25

Lie superior to each kidney (2 of them)
Made up of two seperate glands
Adrenal cortex (outer region has three layers) secretes steroid hormones - the outer layer secretes aldosterone, middle layer secretes cortisol, and the inner layer secretes androgens
The adrenal medulla (central region, part of the sympathetic nervous system) secretes mainly adrenaline (80%)

Question 26

Which layer secretes cortisol?

Answer 26

Middle layer of the adrenal cortex)

Question 27

Which part secretes adrenaline?

Answer 27

The adrenal medulla

Question 28

Adrenal medulla

Answer 28

Secretes adrenaline

Technically the adrenal medulla is a part of the sympathetic nervous system so you have the sympathetic preganglionic neuron when it gets signalled via an action potential, you get action potentials moving down into the adrenal medulla, you get synapsing in the adrenal medulla and get the release of primarily adrenaline

Question 29

The two pathways of adrenaline

Answer 29

Noradrenaline is secreted as part of the sympathetic nervous system’s response to stress

Increase adrenaline (and noradrenaline) secretion from adrenal medulla supplements the actions of the sympathetic nervous system, particularly in times of stress (flight or fright response)

Hormonal pathway - Action potentials travelling down sympathetic preganglionic fibres into the adrenal medulla, in the adrenal medulla we get synapsing using ACh which leads to the release of adrenaline (80%) and noradrenaline (20%) from the adrenal medulla into the bloodstream, these hormones then travel through the bloodstream and often act on the same target cells as those that are impacted by the neural pathway

Neural pathway - Sympathetic preganglionic fibres have action potentials travel down them, and we have synapsing in a sympathetic chain ganglion, through the action of ACh we get action potentials propagating town postganglionic fibres and we get the release of noradrenaline on the target tissue

Noradrenaline is release by both pathways but for the hormonal mechanism it is predominantly adrenaline being release

Question 30

Adrenaline

Answer 30

Water soluble hormone (amino acid derivative) - hormone binds to a cell surface protein, g protein is activated, activation of second messenger pathway and a cellular response in response to that secondary messenger

Cell membrane receptor

Uses a second messenger system

Amplification of cellular response

Acts fast

Question 31

Adrenaline secretion

Answer 31

Stimulus of stress (internal and external forms) this stimulates the hypothalamus which stimulates the sympathetic nervous system’s (if the hypothalamus deems it a stressful enough situation) sympathetic preganglionic fibres which extend from the hypothalamus to send action potentials into the adrenal medulla. This stimulates , through an action potential and the release of ACh, the adrenal medulla secereting adrenaline which travels in the blood to target cells stimulating metabolic effects.

Skeletal muscle and liver - increased breakdown of glycogen to glucose
Fat - increased breakdown of fat (triglycerides) to fatty acids (mobilising energy sources)
Other sympathetic nervous system actions - some systems are activated for physical activity, systems not crucial for short term survival are shut down (repression of activity in places like the kidney and intestine)

The overall effect is that this process makes fuel (glucose and fatty acids) more readily available to cells

There is no negative feedback loop in this process

Sympathetic stimulation can lead to the release of adrenaline from the adrenal medulla

Question 32

Noradrenaline

Answer 32

substance that is released predominantly from the ends of sympathetic nerve fibres and that acts to increase the force of skeletal muscle contraction and the rate and force of contraction of the heart.