Lecture 35 Flashcards
What are the two hormones secreted by the thyroid gland?
Thyroid hormone (increases the basal metabolic rate) and calcitonin (necessary for calcium homeostasis).
What is the thyroid gland location and structure?
The thyroid gland is located just below the larynx on the anterior and lateral surface of the trachea (basically the neck). The structure consists of follicles (small spherical sacs) and each of these follicles is surrounded by follicular cells. Blood vessels are distributed throughout (between follicles) and parafollicular (C cells) lie in clusters between the follicles.
Why is iodine an important part of our diet?
Thyroid hormones can only be stored in the follicles when iodised, without iodine in our diet this can’t be done.
Describe the process of secreting thyroid hormone from the iodised state to the carrier protein.
The thyroid hormone gets released from its iodised state in the follicles in either the active T3 form or the more plentiful T4 form, these will then travel in the blood bound to the carrier protein thyroid-binding globulin until it reaches the target cell.
What are the metabolism affects of thyroid hormone?
Increases body heat production (due to increased oxygen consumption and ATP hydrolysis), stimulates fat mobilisation and oxidation of the fatty acids in many tissues, decreases triglyceried and cholesterol levels.
Stimulates carbohydrate metabolism, enhances insulin dependent entry of glucose into cells, increases rates of gluconeogenesis and glycogenolysis.
Sets the basal metabolic rate.
What is the basal metabolic rate?
The body’s rate of energy expenditure under basal conditions (awake but at physical and mental rest, lying down with no muscle movement at a comfortable temperature and fasted for 12-18 hours).
Why is thyroid hormone important for growth?
It is crucial for proper nervous system formation, hence it will lead to mental problems if there are thyroid deficiencys.
What are the effects in adults of too much/too little thyroid hormone?
Hyposecretion (myxedema): Low metabolic rate, weight gain, cold intolerant, lethargic and slow heart rate, these are caused by not using enough energy.
Hypersecretion (graves disease): Causes high metabolic rate and as such weight loss, heat intolerance, nervousness and increased heart rate, these effects are due to too much energy.
What is a goitre? Why do they occur?
A goitre is an enlargement of the thyroid gland, They due to a lack of iodine, this causes thyroid hormone production is halted. As such the thyroid stimulating hormone secretion from the anterior pituitary is not inhibited and the thyroid stimulating hormone builds up in the thyroid gland, causing the enlargement.
What are the relative levels of calcium in the blood and tissues compared to our bones? What is the concentration set point for Ca2+ in the plasma?
1% Ca2+ in the blood and tissues, 99% in the bones. The set point is roughly (9.2-10.4 mg/dL, 2.2-2.6mmol/L).
What are the main forms calcium is stored as in our body? What cells act to reabsorb or deposit calcium? Where do we lose calcium from our body?
Hydroxyapatite (10 calciums in one molecule) and calcium carbonate (1 calcium). Osteoblasts deposit calcium as hydroxyapatite, osteoclasts reabsorb it into the blood stream. We lose calcium from filtration by the kidneys and fecal loss.
What changes does calcitonin cause which help regulate calcium levels in the bloodstream? What triggers this action?
Calcitonin acts to reduce osteoclast activity and increase osteoblast activity, this returns Ca2+ levels to normal as more Ca2+ is being used up and less is being reabsorbed. Calcitonin secretion is triggered by the C cells detecting high calcium levels.
Where are the parathyroid glands located? What hormone does it release?
On top of the thyroid gland, it releases parathyroid hormone (PTH).
What is parathyroid hormone secretion triggered by? what actions does it cause?
The parathyroid hormones detect a low Ca2+ level and will secrete parathyroid hormone to fix this problem. The effects are increases osteoclast activity (more bone absorbed), reduces osteoblast activity (less bone formed)l more excretion of phosphate in urine (prevents hydroxyapatite formation, further lowering bone formation) and causes less calcium to be excreted in the urine (conserving more calcium.
What are the effects of hypo and hyper calcemia?
Hypocalcemia: increased nervous system excitability leading to muscle tremors, spasms and cramps. This begins when plasma concentration falls to roughly 6mg/dL, at 4 mg/dL the larynx muscles will contract tightly, shutting off air flow and causing suffocation.
Hypercalcemia: nerve and muscle cells become less responsive and excitable at over 12mg/dL depression of the nervous system, emotional disturbances, muscle weakness, sluggish reflexes and occasionally cardiac arrest occur.
