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