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