Thyroid

 

Free T3 (Free Triodothyronine)

FreeT3 is the active thyroid hormone present in serum which regulates metabolism.

Determination of this hormone concentration is important for the diagnostic differentiation of euthroid, hyperthyroid and hypothyroid states.

Free thyroid hormone testing may be ordered along with thyroid antibodies to help diagnose Graves’ disease, an autoimmune disorder that is the most common cause of hyperthyroidism.

Free T4 (Free Thyroxine)

FreeT4 is part of the regulating circuit of the thyroid gland and has an effect on general metabolism. Free T4 is measured together with Free T3 and TSH when thyroid function disorders are suspected.

Since free T4 is the active form of thyroxine, it is thought by many to be a more accurate reflection of thyroid hormone function. This helps the doctor to determine whether the thyroid hormone feedback system is functioning as it should, and the results of the tests help to distinguish between different causes of hyper- and hypothyroidism.

Thyroid hormone tests may be ordered to help evaluate a patient with an enlarged thyroid gland (a goiter) and to aid in the diagnosis of female infertility. If the doctor suspects that the patient may have an autoimmune-related thyroid condition, thyroid antibodies may also be ordered.

Elevated Free T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low Free T4 results may indicate an underactive thyroid gland (hypothyroidism).

The test results alone are not diagnostic but will prompt the doctor to perform additional testing to investigate the cause of the excess or deficiency. Both decreased and increased thyroid hormone results are associated with a variety of temporary and chronic thyroid conditions.

TSH (Thyroid Stimulating Hormone)

TSH is the central regulating mechanism for the biological action of thyroid hormones.
A high TSH result often means an underactive thyroid gland that is not responding adequately to the stimulation of TSH due to some type of acute or chronic thyroid dysfunction.
TSH testing is used to:

• diagnose a thyroid disorder in a person with symptoms,
• monitor thyroid replacement therapy in people with hypothyroidism
• diagnose and monitor female infertility problems,
• help evaluate the function of the pituitary gland (occasionally), and
• screen adults for thyroid disorders (many NHS labs adopt this policy)

The following table summarizes test results and their potential meaning.