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thyroid

thyroid physiology and hormones

introduction

  • the hypothalamus releases thyrotropin releasing hormone (TRH)
  • TRH stimulates the anterior pituitary to release thyroid stimulating hormone (TSH)
  • TSH stimulates the thyroid to produce thyroxine (T4)
  • T4 is converted peripherally to T3 which is more active than T4
  • T3 binds to nuclear receptor proteins that function as transcription factors to regulate the expression of many genes. Nongenomic actions of thyroid hormone also regulate important physiologic parameters.
    • T3 exerts widespread metabolic effects including:
      • increases tissue thermogenesis and thus basal metabolic rate (BMR)
      • reduces serum cholesterol levels
      • reduces systemic vascular resistance
  • T4 inhibits further release of TRH and TSH in a negative feedback loop

testing for thyroid disease

  • patients with hyperthyroidism generally have a low TSH, while those with hypothyroidism generally have a high TSH, thus, for most patients a TSH is all that is needed to screen for thyroid disorder.
  • high pre-test probability patients with a normal TSH should also have a free T4 level performed, while those with suspected hyperthyroidism with a low TSH but normal free T4, should have T3 testing added.
  • screening in patients with atrial fibrillation is low yield with clinical hyperthyroidism being present in ~1% while clinical hypothyroidism is present in ~1.5%1)
1)
Arch Intern Med 156:2221, Oct 28, 1996
thyroid.txt · Last modified: 2014/05/11 12:44 by gary1