FT4

What is FT4?

Free T4 is the thyroid's main hormone, largely a reserve form your body converts into the active version when needed. Think of it as fuel in the tank, ready for use. A stable reserve helps your thyroid regulate your energy and metabolism evenly.

Clinical frame

Free T4 is the non-protein-bound, available fraction of thyroxine and the second step after TSH: when TSH is abnormal, it distinguishes subclinical (free T4 normal) from overt (free T4 abnormal) thyroid dysfunction. A low free T4 with a normal TSH points to central hypothyroidism ^[1].

T4 is a prohormone: about 80% of active T3 arises through peripheral conversion by deiodinases, which determine tissue by tissue how much active hormone a cell receives. Serum free T4 is therefore an approximation of the actual hormone action in the tissues ^[2].

The value matters within the reference interval too: in a meta-analysis of individual data from 134,000 participants, a high-normal free T4 gave roughly 20% more cardiovascular events and mortality than values around the 20th to 40th percentile, especially above the age of 70. Treatment is not indicated for this ^[3].

What moves this value?

The thyroid picture is nuanced and moves through heredity, iodine, stress and sometimes autoimmune processes.

This is mainly a marker to follow, not to adjust yourself. Take an unusual value along as a talking point for your doctor.

What this connects

• TSH

References

1. Chaker, L. et al. Hypothyroidism. Nat Rev Dis Primers 8, 30 (2022). doi.org/10.1038/s41572-022-00357-7
2. Luongo, C., Dentice, M. & Salvatore, D. Deiodinases and their intricate role in thyroid hormone homeostasis. Nat Rev Endocrinol 15, 479–488 (2019). doi.org/10.1038/s41574-019-0218-2
3. Xu, Y. et al. The optimal healthy ranges of thyroid function defined by the risk of cardiovascular disease and mortality: systematic review and individual participant data meta-analysis. Lancet Diabetes Endocrinol 11, 743–754 (2023). doi.org/10.1016/S2213-8587(23)00227-9