Thyroid Flashcards
Where is the thyroid located?
Anterior to the larynx and upper trachea. Inferior to thyroid cartilage.
Describe the structure of the thyroid
Two lateral lobes joined by a central isthmus
Which type of cells are found in the thyroid?
Follicular (contain colloids) and parafollicular (C-cells)
What hormones does the thyroid produce?
Thyroxine (T4), tri-iodothyronine (T3) and calcitonin
Which hormones are produced by the follicular cells and what are their functions?
T3 & T4 - derived from tyrosine with addition of iodine. T3 is the active form, T4 is stable therefore more T4 is released and later converted to T3
Which hormone is released by the parafollicular cells and what is its function?
Calcitonin. Involved in calcium metabolism (mostly in pregnant women)
How are T3 & T4 synthesised?
Transport of iodide into epithelial cells, thyroglobulin produced, exocytosis, oxidation of iodine, iodination of side chain of tyrosine residues to form MIT and DIT, coupling to form T3 & T4 ~1:10
Where are T3 & T4 stored?
Extracellularly in lumen of follicles as part of thyroglobulin molecules
How are T3 & T4 secreted?
Endocytosis. Proteolytic cleavage of thyroglobulin occurs to release them and they diffuse into circulation
How are T3 & T4 transported?
They are hydrophobic and therefore are bound to proteins (TBG, pre-albumin and albumin)
Where is TRH released from?
Dorsomedial nucleus of hypothalamus
Describe TSH
Glycoprotein consisting of two non-covalently linked sub units
What effects do T3 & T4 have on metabolic rate?
Increase metabolic rate - increased glucose uptake, stimulate mobilisation and oxidation of FA, stimulate protein metabolism. Mainly catabolic
Why are T3 & T4 important for growth and development?
Affect bone mineralisation, synthesis of heart muscle protein, development of cellular processes of nerves, hyperplasia of cortical neurones and myelination. Absence = cretinism.
What is the mechanism of action of T3 & T4?
Within the cell, bind to a receptor and cause conformational change that unmasks DNA-binding domain, increase rate of transcription, stimulates oxidative energy metabolism, increases cell activity
How can T4 be converted into T3?
Removal of the 5’-iodide. Helps regulate amount of free hormone.
What does removal of the 3’-iodide produce?
Inactive rT3
What causes Hashimoto’s disease?
Destruction of follicles or production of an antibody that blocks the TSH receptor
How is Hashimoto’s disease treated?
Oral thyroxine - over treating it could lead to hyperthyroidism
What are the effects of hypothyroidism?
Cold intolerance, decreased BMR, mood swings, weight gain, lethargy, bradycardia, NMS weakness, dry/flaky skin, alopecia
Aside from Hashimoto’s disease, what are other causes of hypothyroidism?
Post-surgery, radioactive iodine, anti-thyroid drugs, secondary (lack of TSH), congenital, iodine deficiency
What are the effects of hyperthyroidism?
Heat intolerance, increased BMR, increased oxygen consumption, eye signs, weight loss, hyperactivity, tachycardia, skeletal and cardiac myopathy, osteoporosis
What is Grave’s disease?
Autoimmune disease in which antibodies are produced that stimulate TSH receptors on follicle cells resulting in increased production/release of T3 and T4
How would you treat people who have Grave’s disease?
Carbimazole - inhibits addition of iodine into thyroglobulin
