Hormone
Normal Level
Description
Thyroid Stimulating Hormone (TSH)
0.27 - 4.2 mU/L
  • A measure of plasma TSH which is produced by the pituitary gland in response to thyrotropin-releasing hormone and stimulates production and secretion of thyroid hormones
  • Usually considered the best initial test in the diagnosis of thyroid disease:
    • Elevated levels can suggest hypothyroidism
    • Decreased levels suggest hyperthyroidism
  • Serial measurements can be used to manage thyroid hormone therapy
Free Thyroxine (FT4)
12 - 22 pmol/L
  • A measure of the metabolically active quantity of thyroxine, which circulates unbound to serum proteins
  • Used to determine thyroid over or underactivity in the setting of an abnormal TSH;
    • Elevated levels can confirm thyrotoxicosis
    • Decreased levels can confirm hypothyroidism
Total Thyroxine (T4)
5 to 12 μg/dL
  • A measure of total T4 in the plasma, including both and unbound forms
  • Abnormal levels despite normal thyroid function can be caused by conditions which alter levels of thyroid binding globulin:
    • Drugs or illness can increase the thyroid biding proteins available to carry T4 and increase total levels despite a constant absolute concentration of free "active hormone"
    • An example is pregnancy or hyper oestrogenic states which increase levels of TBG and thus increase levels of total T4
Total Triiodothyronine (T3)
3.1 - 6.8 pmol/L.
  • A measure of the metabolically active quantity of T3:
    • The major source of T3 comes from peripheral conversion of T4
  • Can be used to confirm hyperthyroidism:
    • Hyperthyroidism usually associated with greater increase in T3 than T4
    • Small proportion of patients with hyperthyroidism have a normal T4 but an elevated T3 (so called "T3 toxicosis")
  • Conversely many patients with hypothyroidism have normal T3 levels
  • T3 levels are often decreased in clinically euthyroid patients with non-thyroidal illness (so called "sick-euthyroid syndrome"
  • Difficulty in interpreting abnormal results makes it a poor initial screening test