Serum/plasma tests
Steroid Hormones
Bioavailable testosterone
Dihydrotestosterone (DHT) (TOS)
17-OH progesterone (TOS)
Peptide Hormones
Follicle Stimulating Hormone (FSH)
Insulin (TOS)
Luteinizing Hormone (LH)
Secretin (TOS)
Sex Hormone Binding Globulin (SHBG)
Thyroid Stimulating Hormone (TSH)
Vasoactive Intestinal Polypeptide (VIP) (TOS)
Other Hormones
Free Triiodothyronine (FT3)
Free Thyroxine (FT4)
Triiodothyronine (T3)
Thyroxine (T4)

TOS - To be offered shortly

Tests ›Serum/plasma tests › Hormones › Other Hormones › Thyroxine (T4)

Thyroxine (T4)

Thyroxine is the major hormone produced by the thyroid gland. T4 production is under negative feedback control from the pituitary and the hypothalamus. TSH stimulates T4 production, and above a certain threshold, T4 suppresses TSH secretion by direct feedback on the pituitary as well as by directly inhibiting the release of TRH from the hypothalamus. Thyroid hormones have profound effects on important processes in the body such as development, growth, carbohydrate metabolism and lipid metabolism. Thyroid hormones also have effects on the cardiovascular system (heart rate, contractility), central nervous system (mental state) and the reproductive systems (infertility in hypothyroidism). Thyroxine is mostly bound to Thyroid binding globulin with a small fraction bound to albumin and pre-albumin. Only a small portion (<0.03%) is unbound (free form). Measurement of T4 is most commonly used to establish/rule out diagnosis of hypothyroidism or hyperthyroidism. The test may also be used to assess a patient with an enlarged thyroid gland (goiter) or as for diagnosis and monitoring of female infertility. Levels are decreased in cretinism, myxedema, hypothyroidism, Hashimoto Syndrome and chronic thyroiditis. Elevated levels are encountered in hyperthyroidism, Acute thyroiditis, and hepatitis. Total Thyroxine is elevated in pregnancy or with estrogen treatment, whereas it is decreased with ACTH or Cortisone therapy.

Determination of T4 is performed by Enzyme Immunoassay.

Patient Preparation
Where appropriate and possible, avoid taking thyroid medication 48 hrs prior to specimen collection.

Specimen requirements
Serum specimen - from a plain redtop or a redtop gel (tiger top) tube. Separate serum from cells after centrifugation, if a plain redtop tube is used. Minimum volume required is 1 mL. Specimen should be kept refrigerated where possible.

Shipping Instructions
Ship specimen at ambient temperatures.

Additional information