TOTAL THYROXINE (T4)

Clinical Significance:

Thyroxine (T4) is a hormone synthesized and secreted by the thyroid gland and plays an important role in regulating metabo­lism. Secretion into the circulation is in response to the pituitary hormone TSH and is regulated by a negative feedback mechanism involving the Thyroid gland, pituitary gland and the hypothalamus.

In the circulation, 99.95% ; of T4 is reversibly bound to transport proteins, primarily thyroxine binding globulin (TBG) and to a lesser extent albumin and thyroxine binding prealbumin(TBPA). The remaining T4 is not bound to transport proteins but is free in the circulation. This unbound fraction or free T4 (FT4) is both metabolically active and a precursor to triiodothyronine(T3). Whenever evaluation of T3, T4, TSH fails to give a clear diagnostic picture, it is advisable to do the FT3, FT4 tests. Especially when the patient is on treatment or pregnant, free hormones give a better picture of thyroid status.

Elevated In:
Grave’s Disease

• Toxic Multinodular Goiter
• Toxic adenoma

Transient Hyperthyroidism

• Subacute thyroiditis,
• Hashimoto’s Thyroiditis

Rarely

• Hyperthyroid secondary to pituitary disease
• Thyroid Cancer
• Amiodarone

Decreased Thyroxine or Free T4

• Primary Hypothyroidism
• Hashimoto’s Thyroiditis
• Idiopathic Myxedema
• Previous treatment of Hyperthyroidism
• Sub acute thyroiditis
• Prior neck radiotherapy
• Iodine deficiency or excess
• Congenital
• Medications

  a. Lithium

  b. Sulfonamides

  c. Phenylbutazone

  d. Amiodarone

  e. Thiourea

Secondary Hypothyroidism

• Pituitary dysfunction

Tertiary Hypothyroidism

• Hypothalamic disease
• Very rare Tissue resistance to Thyroid hormone