Total T3

Measurement of total plasma T3 is a sensitive test for hyperthyroidism since levels are usually raised proportionately more than total T4. In T3 thyrotoxicosis, total T3 is elevated but total and free T4 are normal.

Difficulty in interpretation may occur in:

• amiodarone treatment - this inhibits the deiodination of T4 and so causes an increase in free T4 and reduction in free T3
• pregnancy or oral contraceptives - total T3 and T4 are raised as a result of raised thyroixine binding globulin
• severe infection, post myocardial infarction, chronic liver disease, chronic renal failure, post surgery, drugs eg NSAIDs, salicylates, carbamazepine, phenytoin, propranolol - all may lower total T3

Measurement of free T3 may be useful when a false low or high T3 or T4 is suspected.