Physiologic Basis
Total T4 is a measure of thyroid gland secretion of T4, bound and free, and thus is
influenced by levels of thyroid hormone binding proteins. Only free T4 is biologically active.
TSH is an anterior pituitary hormone that stimulates the thyroid gland to produce
thyroid hormones. Secretion is stimulated by thyrotropin releasing hormones from the hypothalamus.
There is negative feedback on TSH secretion by circulating thyroid hormone.
T3 is the primary active thyroid hormone. Approximately 80% of T3 is produced by
extrathyroidal deiodination of T4 and the rest by thyroid gland. Total T3 is influenced by levels of
thyroxine binding proteins.
Patterns of Thyroid Function Tests in Patients with Thyroid Disease
Type of disease
|
T4
|
T3
|
TSH
|
Conventional hyperthyroidism (95%) |
Raised |
Raised |
Undetectable |
T3
hyperthyroidism (5%) |
Normal |
Raised |
Undetectable |
Subclinical hyperthyroidism |
Normal |
Normal |
Undetectable |
Primary hypothyroidism |
Low
|
Not
indicated |
Raised |
Subclinical hypothyroidism |
Normal |
Not
indicated |
Raised |
Secondary hypothyroidism |
Low
|
Not
indicated |
Undetectable |