T4 (Thyroxine)
T4 is the primary hormone secreted by the thyroid gland, accounting for about 80% of thyroid hormone output. T4 itself is relatively inactive — it serves as a reservoir that is converted to the active form (T3) as needed in tissues. Total T4 measures both protein-bound and free forms.
Reference Ranges
Male
5 – 12
µg/dL
Female
5 – 12
µg/dL
Child
6 – 13
µg/dL
Ages 1–12
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
Low T4 with high TSH confirms hypothyroidism. Causes include Hashimoto's thyroiditis, thyroid surgery, radioactive iodine treatment, iodine deficiency, and certain medications (lithium, amiodarone).
What High Levels Mean
High T4 with low TSH confirms hyperthyroidism. Causes include Graves' disease, toxic nodular goiter, thyroiditis, and excess thyroid medication. High total T4 with normal TSH may simply reflect elevated binding proteins (pregnancy, estrogen therapy).
Frequently Asked Questions
Why test free T4 instead of total T4?
Free T4 is unaffected by protein levels that can change total T4. Pregnancy, birth control pills, and liver disease increase thyroid-binding proteins, raising total T4 without increasing active hormone. Free T4 gives a more accurate picture of thyroid function.
Can iodine deficiency cause low T4?
Yes. Iodine is essential for making thyroid hormones. Iodine deficiency is the most common cause of hypothyroidism worldwide, though it is less common in India since salt iodization became mandatory. Coastal populations rarely have deficiency.
What happens if I take too much thyroid medication?
Excess levothyroxine raises T4 and T3 while suppressing TSH. Symptoms include rapid heartbeat, anxiety, weight loss, insomnia, and bone loss (osteoporosis). This is why regular TSH monitoring is essential for anyone on thyroid medication.
This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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