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
Thyroid Function Test interpretation

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 700 6

Critical Care

Compendium

Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM with a passion for rugby; medical history; medical education; and informatics. Asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | vocortex |

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