T4 (THYROXINE)

• 4.5-12 ug/dl

Paediatric normal range- 

• 1-3 days- 11.8-22.6 ug/dl
• 1-4 months-7-2-14.4 ug/dl

• Thyroxine (T4) is one of the two major hormones produced by the Thyroid Gland. Almost all of the T4 found in the blood is bound to protein, the rest is free (Unbound) and is the biologically active form of the hormone. Most of the hormones produced by the Thyroid is T4. It is relatively Inactive, but it is converted into much more active T3 in the liver and other tissues.
• If the Thyroid releases excess amounts of T4 and T3, there will be symptoms associated with Hyperthyroidism (Rapid heart rate, weight loss, nervousness, hand tremors, irritated eyes, difficulty sleeping). Graves disease is the most common cause of Hyperthyroidism.
• If there is decreased production of Thyroid hormones, there will be symptoms associated with Hypothyroidism (weight gain, dry skin, constipation, cold intolerance , fatigue). Hashimoto Thyroiditis is the most common cause of Hypothyroidism.
• T4 test is used to diagnose thyroid dysfunction in adults. Elevated T4 level are characteristically seen in patients with overt Hyperthyroidism.

• When there are s/s of Hyperthyroidism (Increased heart rate, Anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, Menstrual irregularity

• Hyperthyroid states
• Grave’s disease
• Toxic multinodular goiter
• Sub-acute or chronic thyroiditis
• Early Hashimoto’s disease
• Iodine-induced hyperthyroidism
• Germ cell tumours
• Trophoblastic disease

• Hypothyroidism (including later stage of Hashimoto’s disease, cretinism, myxedema, goitrous disease, following radio iodine or thyroid ablation, infiltrative diseases such as scleroderma, amyloid goiter)
• TBG deficiency
• Excessively treated Hyperthyroidism
• Inadequately treated hypothyroidism
• Malnutrition
• Systemic illness
• Fasting