Session 10

Question 0

Describe the chemical structure of the thyroid hormones and the mechanisms of their production, storage and secretion

Answer 0

Production: T3 & T4 (derived from tyrosine with addition of iodine) calcitonin (polypeptide)
Storage: T3 & T4 stored extracellularly in lumen of follicles
Synthesis: transport of iodine against concentration gradient, synthesis of thyroglobulin, secretion of thyroglobulin into lumen, oxidation of iodine to produce iodinating species, iodination of side chains of tyrosine residues in thyroglobulin to form MIT and DIT, coupling of DIT with MIT or DIT to form T3 and T4

Question 1

Describe the location and structure of the thyroid gland

Answer 1

The gland is located in the neck in front of the lower larynx and upper trachea
Two nerves - recurrent and superior laryngeal
Three arteries and veins (superior, middle, inferior)
Butterfly shape - two lateral lobes and a central isthmus
Two major cells - follicular and parafollicular (C) cells

Question 2

Describe how activity of the thyroid gland is controlled

Answer 2

Under control by hypothalamus and anterior pituitary gland
Hypothalamus (dorsomedial nucleus) - thryrotrophin releasing hormone (TRH) released due to circulating levels of T3,T4 (negative feedback), stress (increase release), temperature (low increases release) 
Anterior pituitary (thyrotrophs) - thyroid stimulating hormone (TSH)

Question 3

Describe the effects of thyroid hormones on cells and the body as a whole

Answer 3

T3 & T4 increase metabolic rate, stimulate glucose uptake, metabolism and oxidation of fatty acids and protein metabolism
Generally catabolic - increase in BMR, heat production and oxygen consumption
Important for normal growth and development (particularly CNS), directly affect bone mineralisation, increase synthesis of heart muscle protein
Lack of thyroid hormones from birth to puberty = cretinism (severe mental and physical retardation)
Absence = no ovulation

Question 4

Describe the consequences of over- and under- secretion of thyroid hormones

Answer 4

Over secretion (Grave's disease):
Autoimmune - antibodies stimulate TSH receptors on follicle cells --> increased production of T3&T4 - heat intolerance, weight loss, tachycardia, increased appetite and bowel movement, exophthalmos
Treated with Carbimazole - inhibits incorporation of of iodine into thyroglobulin, inhibits thyroid peroxidase (prevents iodination and coupling of tyrosine residues on thyroglobulin), radioactive iodine, surgery
Under secretion (Hashimotos'):
Autoimmune - destruction of thyroid follicles or production of an antibody that blocks TSH receptor on follicle cells - cold intolerance, weight gain, bradycardia, constipation, poor concentration/memory, dry skin, brittle nails, hair loss
Treated with oral thyroxine (T4) - gradually increase dose