Thyroid function tests

The laboratory assessment of hypo- and hyperthyroidism has been simplified by the development of sensitive assays for:

• TSH
• free T3 and free T4

Some laboratories still measure total T3 and total T4 and in such cases some estimate of the concentration of thyroid binding protein concentrations must be made.

Some typical reference ranges in adults are:

• TSH - 0.4 – 4.5 mU/L
• FT4 - 9.0 – 25 pmol/L
• FT3 - 3.5 – 7.8 nmol/L
• TT4 - 60 – 160 nmol/L
• TT3 - 1.2 – 2.6 nmol/L (1)

Note that reference ranges may vary between laboratories.

NICE state:

• ask adults, children and young people with suspected thyroid dysfunction about their biotin intake because a high consumption of biotin from dietary supplements may lead to falsely high or low test results

Reference:

• (1) Association for Clinical Biochemistry (ACB), British Thyroid Association (BTA), British Thyroid Foundation (BTF) 2006. UK guidelines for the use of thyroid function tests
• (2) NICE (October 2023). Thyroid disease: assessment and management