Total Thyroxine (T4)
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.
Free Thyroxine (Free T4)
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.
Thyroid Stimulating Hormone (TSH)
The most important regulator of thyroid homeostasis is TSH. This test is particularly of importance in detecting subclinical thyroid diseases.