When is it prescribed?
More than 99 percent of thyroxine in the blood is bound to proteins, and only the free fraction is biologically active. If the amount of binding protein changes, total T4 and T3 rise or fall, while free T4 and T3 remain normal and the person is euthyroid. TSH increases during pregnancy, taking estrogens and oral contraceptives, and liver diseases; decreases with nephrotic syndrome, taking androgens and glucocorticoids, as well as with congenital deficiency. The analysis is prescribed when total and free hormones contradict each other, as well as when a hereditary disorder is suspected - this avoids an erroneous diagnosis of thyrotoxicosis or hypothyroidism and unnecessary treatment.
How to prepare to donate blood
- on an empty stomach 8–12 hours, you can drink water
- Eliminate alcohol and intense exercise per day
- no smoking in an hour
- donate blood before physiotherapy, x-rays and massage
- Take medications in advance by agreeing with your doctor
How to read the result
Reference values depend on the method and equipment of the laboratory, so it is correct to compare results only within the same laboratory. A deviation of a few percent from the normal limit in itself rarely matters - the overall picture and dynamics are important.
The analysis must be interpreted by a doctor: an isolated indicator without a clinical context is often misleading and leads to unnecessary examinations.