Thyroid Function Tests

• Types of thyroid disorders

  • Hyperthyroidism
  • Hypothyroidism
• Thyroid function tests can be used to check for abnormal thyroid function and determine the cause
• Hormone Tests for thyroid function

  • Thyroid-stimulating hormone (TSH)
  • Triiodothyronine (T3)
  • Thyroxine (T4)
• Primary hyperthyroidism is where the thyroid behaves abnormally and produces excessive thyroid hormones
• Primary hyperthyroidism


  TSH is suppressed by the high T3 and T4, causing a low TSH level
• Secondary hyperthyroidism is where the pituitary behaves abnormally and produces excessive TSH (e.g., pituitary adenoma), stimulating the thyroid gland to produce excessive thyroid hormones
• Secondary hyperthyroidism


  TSH, T3, and T4 will all be raised
• Primary hypothyroidism is where the thyroid behaves abnormally and produces inadequate thyroid hormones. Negative feedback is absent, resulting in increased production of TSH. TSH is raised, and T3 and T4 are low
• Secondary hypothyroidism is where the pituitary behaves abnormally and produces inadequate TSH (e.g., after surgical removal of the pituitary), resulting in under-stimulation of the thyroid gland and insufficient thyroid hormones
• Secondary hypothyroidism


  TSH, T3, and T4 will all be low
• Thyroid disorders and hormone levels

  • Primary Hyperthyroidism
  • Low TSH, High T3 and T4
  • Secondary Hyperthyroidism
  • Raised TSH, T3, and T4
  • Primary Hypothyroidism
  • Raised TSH, Low T3 and T4
  • Secondary Hypothyroidism
  • Low TSH, T3, and T4
• Abnormal pituitary behavior and inadequate TSH production

  Result in under-stimulation of the thyroid gland and insufficient thyroid hormones
• Levels of TSH, T3, and T4 in Primary Hyperthyroidism

  • TSH: Low
  • T3: High
  • T4: High
• Levels of TSH, T3, and T4 in Secondary Hyperthyroidism

  • TSH: High
  • T3: High
  • T4: High
• Levels of TSH, T3, and T4 in Primary Hypothyroidism

  • TSH: High
  • T3: Low
  • T4: Low
• Levels of TSH, T3, and T4 in Secondary Hypothyroidism

  • TSH: Low
  • T3: Low
  • T4: Low
• Antibodies related to thyroid gland

  • Anti-thyroid peroxidase (anti-TPO) antibodies
  • Anti-thyroglobulin (anti-Tg) antibodies
  • TSH receptor antibodies
• Imaging techniques for thyroid diagnosis

  • Ultrasound
  • Radioisotope scans
• Radioactive iodine is used to investigate hyperthyroidism and thyroid cancers
• Iodine is used by thyroid cells to produce thyroid hormones
• Gamma camera detects gamma rays emitted from radioactive iodine to give functional information about the thyroid gland
• Findings in Radioisotope scans

  • Diffuse high uptake in Grave’s Disease
  • Focal high uptake in toxic multinodular goitre and adenomas
  • “Cold” areas can indicate thyroid cancer