This description aims to provide patients with some useful information on the tests which may be included as part of their health screening. The tests are explained briefly. Because reference ranges are typically defined as the range of values of the median 95% of the healthy population, it is unlikely that a given specimen, even from a healthy individual, will show “normal” values for all tests. All results should be correlated with patient’s history and clinical findings. Therefore, your physician is the best person to interpret your laboratory results. Always consult your physician.
Thyroxine is produced exclusively by the thyroid. Thyroxine in the blood circulation exists in two forms: 99.95% bound to plasma proteins and 0.05% unbound. Total T4 level is a good measurement of the thyroid function if thyroxine-binding globulin (TBG) is normal. High total T4 in hyperthyroidism, states with increased TBG (e.g. pregnancy, oestrogen medication), some cases of acute thyroiditis, hepatitis and obesity. Reduced total T4 in hypothyroidism, states with decreased TBG (e.g. nephrotic syndrome, chronic liver disease, malnutrition) and strenuous exercise.
The 0.05% of unbound T4 can be measured as Free T4. This is the biological active form and replaces the Free Thyroxine Index (FTI), which is a mathematical calculation to provide an estimate of FT4. FT4 level is useful in patients with altered TBG, including those with acute and chronic illnesses and pregnancy.
The most important regulator of thyroid homeostasis is TSH. This test is particularly of importance in detecting subclinical thyroid diseases.