Thyroxine

Total thyroxine (T4) measures the full amount of thyroxine in your blood, including both the portion bound to carrier proteins and the much smaller unbound (free) fraction. Thyroxine is the main hormone produced by your thyroid gland and serves as the precursor to the more biologically active T3. Because the vast majority of T4 is bound to proteins like thyroxine-binding globulin, the total T4 reading is heavily influenced by protein levels. Changes in binding protein (from pregnancy, oestrogen, liver disease, severe illness, or certain medications) can shift the total T4 without reflecting a real change in thyroid function.

For that reason, most UK laboratories prefer free T4 (FT4) as the primary thyroid monitoring test alongside TSH. Total T4 is still useful in specific clinical contexts, such as monitoring the response to levothyroxine, screening for congenital hypothyroidism, or interpreting results where binding-protein abnormalities are suspected. Think of total T4 as the total hormone in circulation, including what is already tied up in transit, whereas free T4 is the active fraction ready to be taken up by tissues.

Your total T4 level is influenced by the health of your thyroid gland, your binding-protein levels, and the medications you take. Oestrogens (HRT, the combined oral contraceptive, pregnancy) typically raise total T4 by increasing thyroxine-binding globulin. Corticosteroids, androgens, and severe illness can lower it. If your total T4 falls outside the expected range, your GP will usually interpret it alongside TSH and free T4 to decide whether any further assessment is warranted.