Thyroid Function Tests

TRH: Thyrotropin releasing hormone (tripeptide)

• Secreted by hypothalamus
• TRH stimulates the release of TSH from the pituitary
• Inhibited by high levels of T3 and T4
• Stimulated by low levels of T3 and T4
• Abnormalities in production are rare

TSH: Thyroid stimulating hormone (polypeptide)

• Released from anterior pituitary
• Increases production and release of thyroxine (T4 and T3 form the thyroid)
• Inhibited by high levels of T3 and T4
• Stimulated by high levels of TRH and low levels of T3 and T4

T4 : Thyroxine

• Produced only by the thyroid gland
• Travels in plasma bound to protein or in free form
• Converted to T3 (active) and RT3 (inactive) peripherally in blood and tissues

T3: Triiodothyronine

• 85% is produced form converted T4
• 15% is directly secreted from thyroid
• 5 times as active as T4

TBG: Thyroid binding globulin

• Allows carriage of T3 and T4 in blood
• Inactive when combined with TBG (active component free T3 and T4)
• Increase:
  • Pregnancy, OCP, hepatitis, hypothyroid
  • Phenothiazines, opiates
• Decrease:
  • Thyrotoxicosis, hypoalbuminemia
  • Steroids, phenytoin, NSAID

Interpretation of TFT

• Basic investigations of thyroid function
  • Hyperthyroidism: Best test is plasma T3 (raised)
  • Hypothyroidism: Plasma TSH (raised) plasma T4 (lowered)
• Problem:
  • Cheapest test is to measure the total plasma levels of T3 and T4
  • False high and low readings secondary to TBG alterations
• Solution:
  • More expensive tests with TBG and free T3 and T4
  • Only the free T3 and T4 are the active elements

Basic tests

• Plasma T4 – Total thyroxine
  • Collected at any time from uncuffed arm
  • False high: Excessive TBG (hereditary, Pregnancy and oestrogens)
  • False low: Salicylates, NSAID, phenytoin, steroids, carbamazepine, TBG deficiency
• Plasma T3
  • False high: Pregnancy and oestrogens
  • False low: Infection, surgery, AMI, chronic liver disease, CRF, propanolol, NSAID, carbamazepine, phenytoin, steroids
• Plasma TSH
  • Indication – Suspect hypothyroidism
• Free T3/T4
  • Indication – Useful when suspect false high/low plasma T3/4 readings
• Free T3/T4 index
  • Reflects alterations in protein binding

Remember:

• Anterior pituitary (Highest blood flow of any tissue in body)
  • Release of ACTH, GH, FSH, LH, PRL, TSH
• Posterior pituitary
  • Release of ADH, oxytocin

References and Links

• CCC – Medications and thyroid function