ENDOCRINOLOGY (thyroid gland)

Thyroid hormone
1. Released in response to thyroid stimulating hormone from the anterior pituitary which in turn is stimulated by thyroid regulating hormone from the hypothalamus
2. Include T3 (triiodotyrosine) and T4 (thryoxine)
  1. Made in follicles of the thyroid gland made up of follicular cells surrounded by a central, protein rich colloid
    1. Between these are C cells that synthesise and secrete calcitonin. Calcitonin is secreted when calcium levels are high. Inhibits osteoclast and stimulates calcium excretion by the kidneys. This is important for growth in children
      1. Parathyroid glands are made up of chief calls that secrete parathyroid hormone when calcium levels are low
        PTH mobilises calcium from bone by increasing osteoclast activity and increasing growth factor RANKL. Enhances calcium reabsorption at the kidneys.
        Stimulates calcitriol that enhances calcium and phosphate absorption from the digestive tract. Calcitriol is synthesised from D3 in the kidneys
        High calcium levels stop PTH by negative feedback
        Hyperparathyroidism causes hypercalcemia. Symptoms include fatigue, confusion, calcification of kidneys and soft tissue. Treatments involves parathyroid removal, calcitonin administration and hypertonic fluid
        Hypoparathyroidism causes hypocalcaemia. Symptoms include muscle spasms, weak heart beat and osteoporosis. Treatments involve calcium supplements and vitamin D
      2. Normal calcium levels = 2.3-2.4mmol/l (cells) 1.4mmol/l (plasma)
    2. Thyroglobulin proteins become iodinated to monoiodotyrosine and diiodotyrosine from iodine in the diet and these join to form T3 or T4. These are taken into follicular cells at the apical surface
      1. T4 is produced in greater concentrations than T3 but T3 is four times more poten. T4 is therefore converted to T3 by liver or target tissue
      2. TSH binds to Gs receptors. Protein kinase sequences cause the release of T3 and T4 from thyroglobulin to the blood at the basolateral surface
        TSH secretion stops when levels of T4 become high - negative feedback
      3. Thyroid hormones are lipophilic and so bind to intracellular receptors on the nucleus of mitochondria
        This causes increased protein synthesis, cell growth and maturation, increased metabolic rate, increased oxygen consumption, increased cardiac output, heart rate and force of contraction, thermogenesis and increased sensitivity to sympathetic stimulation
        In children this causes a rise in body temperature - survival technique
      4. Hypothyroidism - underactive thyroid
        Caused by the immune system attacking the thyroid gland. This causes fatigue, weight gain, feeling cold, dry skin and hair, heavy periods, constipation and slowed thinking
        May be central (low TSH) or primary (low T4 levels)
        Treatment includes giving thyroxine and T3 orally. Overdose can lead to angina, cardiac dysrhythmia or hyperthyroidism
      5. Hyperthyroidism - overactive thyroid
        Can be due to Grave's disease which is also associated with Graves opthalmopathy
        Symptoms include shaking, nervousness, irritability, rapid heart beat, feeling hot, weight loss, fatigue, frequent bowel movements and fatigue
        Treatment involves beta blockers such as propranolol to reduce heart palpitations, anti thyroid drugs such as carbamizole which affects the synthesis, propylthiouracil and methimazole
        May also be treated with iodine

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ENDOCRINOLOGY (thyroid gland)

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	Erstellt von Alix Stonehouse vor mehr als 9 Jahre