FrT4 ( FREE THYROXINE)

• 0.7 -1.8 ng/dl

• Thyroxine (T4) is a hormone synthesized and secreted by the thyroid gland. In circulation 99.95 % of T4 is reversibly bound to transport protein, primarily Thyroxine binding Globulin (TBG) and to a lesser extent albumin and thyroxin binding pre albumin (TBPA). The remaining T4 is not bound to transport proteins but it is free in the circulation. The unbound remaining T4 or Fr T4 is metabolically active and precursor of T3
• Whenever the thyroid function tests i.e. T3 ,T4,TSH fails to give a diagnostic conclusion ,a combination of FT4 and FT3 is checked especially when the patient is on treatment or pregnant, free hormone give a better picture of thyroid status

• When there is s/s of Hyperthyroidism (Increased heart rate, Anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, and menstrual irregularity)

Graves disease

• Toxic multinodular goiter
• Toxic adenoma

Transient hyperthyroidism

• Subacute thyroiditis
• Hashimoto’s thyroiditis

Rarely

• Hyperthyroid secondary to pituitary disease
• Thyroid cancer

Primary hypothyroidism

• Hashimoto’s thyroiditis
• Idiopathic myxedema
• Previous treatment of hyperthyroidism
• Sub-acute thyroiditis
• Prior neck radiotherapy
• Iodine deficiency or excess
• Congenital
• Medications- Lithium, Sulfonamides , Phenylbutazone , Amiodarone ,Thiourea

Secondary hypothyroidism

• Pituitary dysfunction

Tertiary hypothyroidism

• Hypothalamic disease- Very rare
• Tissue resistance to thyroid hormone