LAB DIAGNOSIS OF THYROID DISORDERS/THYROID FUNCTION TESTS

 Thyroid disorders are analysed using the Thyroid function tests/Thyroid Profile

1. T3/T4 assay - 

• Total T3/T4 & Free T3 & T4 - analysed by radioimmunoassay(RIA) or ELISA or CLIA(chemiluminescence assay) or Fluorescent immunoassay(FIA)
• Normal Level 
• T3 - 1.8 - 3 nmol/L
• T4 - 65-150 nmol/L
• f T3 - 0.3% of total T3
• f T4 - 0.03% of total T4

2.Plasma TSH

• Normal level - 0.5-5 mU/L
• TSH is produced by pituitary gland 
• its secretion is regulated by the feedback mechanism of fT3 & fT4 secretion

3.Thyroid Antibodies

• Thyroid stimulating immunoglobulin(TSIg) is the antibody generated against the TSH receptor
• Measurement of TSIg  help in the diagnosis of Graves disease.
• Antibodies to thyroid peroxidase - Measurement of Anti-TPO antibodies help in the diagnosis of Hashimoto's thyroiditis(autoimmune thyroiditis)

4.Radioactive Iodine uptake

• To study the thyroid structure & function
• Radioactive iodine is given and the radio scan of thyroid gland is done to check for the amount of iodine taken up by the gland.
• Special diagnostic value to detect cancerous lesion

5.TRH Response Test

• Detects the abnormality in hypothalamic- pituitary axis
• Normal scenario - TRH administration increases TSH production by pituitary in turn increasing the amount of T3 & T4
• Hyperthyroidism - TRH administration has no effect on  TSH production as the T3/T4 levels are already high and the negative feedback effect of T3/T4 overpowers the stimulant effect
• Hypopituitarism  - TRH administration has no effect on TSH production and the T3/T4 levels are low
• Hypothyroidism - TRH administration has exaggerated effect on TSH production causing increased TSH levels but low T3/T4 levels due to defect in thyroid gland.