FT3 is normally tested only when which two test results are observed?

• A. TSH is high and FT4 is normal
• B. TSH is low and FT4 is high
• C. FT4 is high and T3U is elevated
• D. TSH is high and FT3 is high

Answer: (A)

When evaluating thyroid function, the role of free T3 testing is most relevant when the results of the main screening tests are discordant—specifically, when the pituitary signal is elevated (TSH is high) but the circulating thyroid hormone level in the blood that matters for tissue effect (free FT4) is still normal. This pattern is called subclinical hypothyroidism. In that situation, checking free T3 helps gauge whether there is reduced peripheral thyroid hormone action despite a normal FT4, and whether the patient might be at risk of progressing to overt hypothyroidism or whether symptoms could be explained by other factors. So FT3 is typically considered in this scenario to add information beyond TSH and FT4 before deciding on management.