thyroid gland

Question 1

Explain the importance of dietary iodine for hormone synthesis and the consequences of deficiency

Answer 1

• the function of the thyroid gland is to take iodine, found in many foods, and use it to synthesize thyroid hormones: thyroxine (T4) and triiodothyronine (T3).
  
  • Follicular cells release and store thyroid hormone.
  • Parafollicular cells - also called C cells, secrete calcitonin (role in Ca2+ metabolism not related to major thyroid hormones)
  • The thyroid gland Synthesises hormones to increase the metabolic rate in the body.
  • Almost all the cells will respond to these hormones
  • This will also increase the body temperature due to the increased metabolic rate
  • This gland is able to regulate our temperature
  • Deficiency of I- in diet would mean less T3 an T4 is made

Question 2

Describe the mechanism of release of the hormones and conversion of T4 to T3 in periphery

Answer 2

• T3 and T4 are lipid soluble so can diffuse out the cel but need to bound to a transported protein (thyronine binding globulin (TBG) and albumin) to be transported in the blood. This is needed to maintain a stable pool of T3 and T4
  
  • T4 goes to different cells to be converted to T3.The cells have enzymes thy deiodinase T4 to T3 to make it active

Question 3

Describe the mechanisms, including negative feedback, that maintain hormone levels

Answer 3

• Stress and cold can trigger the hypothalamus to create TRH which causes the relace of TSH from the anterior pituitary gland
  
  • Binding of TSH (from the anterior pituitary) to its receptors on thyroid cells stimulates synthesis of the iodine transporter, thyroid peroxidase and thyroglobulin.
  • The magnitude of the TSH signal also sets the rate of endocytosis of colloid - high concentrations of TSH lead to faster rates of endocytosis, and hence, thyroid hormone
  • release into the circulation. Conversely, when TSH levels are low, rates of thyroid hormone synthesis and release diminish.
    ○ TSH will cause the thyroid gland to produce T3 an T4 to increace the rate of metabolism
    ○ T3 and T4 will go to the hypothalamus and cause secretion of TRH to stop
  • Need to memorise and understand this table
    ○ Thyroid just responds to the TSH from the pituitary in the 1st 2 examples
    ○ Thyroid has the problem in the 2nd 2 examples not the pituitary
    § Pituitary responds correctly
    ○ From this blood test you can diagnose where the problem is ( results are all different)

Question 4

Describe the hormone action in the target tissue and its importance for normal growth and development and for maintenance of basal metabolism

Answer 4

• Hyperthyroidism – An overactive thyroid.
  ○ Fatigue
  ○ Heat intolerance
  ○ Sweating
  ○ Weight loss
  ○ Hyperphagia- over eat due to high metabolic rate
  ○ Nervousness
  ○ tachycardia
  ○ Tremor
  ○ Can cause graves’ disease, toxic nodular goitre
  § graves’ disease- caused by antibodies binding to the TSH rector which drives T3 and T4 creation. Eyes bulge out of their head. If not treated they won’t be able to close their eyes, cornea will dry and go blind. They may also rip the optic nerve
  ○ Diagnosed by a high T4 and low TSH level-see table
  ○ Treaded by: radioactive iodine (Radioactive iodine will destroy cells as all iodine goes into the thyroid), anti-thyroid drugs, surgery….
  
  • Hypothyroidism – An underactive thyroid.
    ○ General tiredness/lethargy
    ○ Weight gain – but poor appetite
    ○ Hair sparse/coarse
    ○ Skin puffy/dry
    ○ Voice husky
    ○ Cognitive impairment
    ○ Depression
    ○ Poor tolerance of cold
    ○ Possible goitre
    ○ lower Cardiac output and slow pulse
    ○ Myxoedema
    § (skin swelling/inflammation)
    ○ Treatment: thyroxine replacement/injections
    ○ Mostly caused by an I- diet deficiency in some countries use of iodized salt Hashimoto’s thyroiditis is the most common cause. (Autoantibodies against thyroglobulin or TPO cause progressive destruction of gland).
    ○ Also, a side effect of treatment for an overactive thyroid or for thyroid cancer.
    ○ Rare – insufficient secretion of TSH from pituitary
    ○
  • Goiter – An enlarged thyroid (at least 3x) Indication of disease but not underlying cause. Can be caused by hyper- or hypothyroidism
  • Thyroid Nodules – Lumps in the thyroid gland.
  • Thyroid Cancer – Malignant thyroid nodules or tissue.
  • Thyroiditis – Inflammation of the thyroid