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
Critical Care
Compendium
BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Associate Professor Curtin Medical School, Curtin University. Emergency physician, Sir Charles Gairdner Hospital. Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |