Flashcards in 19. The Thyroid Gland Deck (33)
What is the main function of the thyroid?
Formation and storage of thyroid hormone.
What percentage of the thyroid is colloid?
What does colloid contain?
What do C cells of the thyroid secrete?
What is the normal blood supply to the thyroid compared to enlarged thyroid glands?
30-60ml/min normally, 100ml/min in enlarged thyroids.
What is the effect of sympathetic stimulation on the thyroid gland?
Increased secretion of thyroid hormone.
Explain the hypothalmic-pituitary-thyroid axis.
The hypothalamus secretes TRH (thyrotropin releasing hormone) which binds to Gq/11 receptors on the pituitary gland, this signals an increased release of TSH (thyrotropin stimulating hormone). TSH binds to TSH receptor on the basolateral membrane of thyroid follicular cell. This leads to stimulation of iodine uptake, production and release of idothyronines and promotion of thyroid growth
What is the short loop of inhibition of the thyroid gland?
High T3/4 causes negative feedback on the pituitary gland so it secretes less TSH and less T3/4 is produced.
What is the long loop of inhibition in the thyroid gland?
High T3/4 levels inhibit the hypothalamus so it releases less TRH, this reduces secretion of TSH and therefore lowers T3/4 production.
What would high TSH levels suggest?
An under active thyroid.
How does uptake of iodine in the thyroid work?
Follicular cells concentrate iodine using an iodine trap. The iodine is dependent on the diet.
How is iodine activated in the thyroid gland?
By peroxidase enzyme.
What are the full names of the hormones produced by the thyroid?
Tetra-iodothyronine/ thyroxine (T4) and tri-iodithyronine.
Why are different amounts of T3 and T4 made?
More T4 is made as it has a longer half life but T3 is more potent. T4 is converted to T3 by deiodination for action.
What percentage of thyroid hormones are bound?
What do thyroid hormones bind to?
Mostly to thyronine binding globulin (TBG) but a little to albumin and pre-albumin.
How are T3/4 degraded?
By the removal of iodine by the liver and kidney.
Why is the half life of T4 greater than that of T3?
Because it has a greater affinity for binding proteins than T3.
What are the general effects of thyroid hormones?
Increase basal metabolic rate by increasing the size and number of mitochondria, increasing oxygen consumption and heat production and increasing nutrient utilisation. It also stimulates most metabolic pathways, promotes normal growth and development of tissues, increases responsiveness of tissues to sympathetic nervous system and various hormones.
How do thyroid hormones affect the nervous system?
Increased myelination of nerve fibres, increased speed of reflexes and increased mental activity.
How do thyroid hormones affect the cardiovascular system?
They increase cardiac output.
How do thyroid hormones affect the skin and subcutaneous tissue?
Increased turnover of proteins and glycoproteins. Without thyroid hormones, the skin becomes saggy.
What does hypothyroidism in new borns cause? Give details.
Cretinism. Severe mental retardation due to failure in CNS development, coarse features and a protruding tongue, diminished linear growth and delayed sexual development.
What are the signs and symptoms of hyperthyroidism?
Heat intolerance, increased perspiration,weight loss, tachycardia, increased bowel movements - including diarrhoea, nervousness, irritability, emotional lability, hyper reflective - tremor of hands and eye signs of exophthalmos (eyes stick out).
What is the major cause of hyperthyroidism?
Grave's disease, an autoimmune disease that stimulates the TSH receptor on follicle cells.
How is hyperthyroidism treated?
Carbimazole to inhibit incorporation of iodine into thyroglobulin, radioactive iodine or surgery to remove the thyroid.
Why must the surgeon be careful when performing a thyroidectomy?
The parathyroid glands are on the outer capsule of the thyroid so they mustn't remove them otherwise the patient won't be able to control Ca levels.
What causes exophthalmos?
Accumulation of adipose tissue behind the eye in Graves' disease.
What are the signs and symptoms of hypothyroidism?
Cold intolerance, decreased perspiration, mild weight gain, bradycardia, constipation, mood swings - anxiety and depression, poor concentration, poor memory, oedema, dry skin, brittle nails and some hair loss.