Thyroxine (T4)

Understanding Thyroxine (T4) and FT4 Testing

Thyroxine, a vital hormone produced by the thyroid gland, plays a crucial role in regulating metabolic processes and energy consumption in the body. This article explores the significance of thyroxine, its role in thyroid function, and the diagnostic value of free thyroxine (FT4) testing.

Thyroxine Function and Regulation:
Thyroxine exists in two forms in the bloodstream: free and protein-bound. It is secreted in response to thyroid-stimulating hormone (TSH) released by the pituitary gland. Dysfunction in thyroxine synthesis can lead to various thyroid disorders, including hypothyroidism and hyperthyroidism.

Symptoms and Effects of Thyroxine Imbalance:
Insufficient thyroxine production results in hypothyroidism, characterized by symptoms like weight gain, fatigue, and menstrual irregularities. Conversely, elevated thyroxine levels lead to hyperthyroidism, manifesting as weight loss, anxiety, and tremors.

Medical Thyroid Tests:
Thyroid tests serve multiple purposes, including diagnosing inherited hypothyroidism, detecting infertility causes, monitoring thyroid dysfunction, and conducting routine screenings for thyroid diseases and pregnancy-related conditions.

FT4 Test Description:
The FT4 test, performed through chemiluminescent analysis, measures the level of free thyroxine in the blood. Patients must adhere to specific preparation steps, including fasting and medication discontinuation, to ensure accurate results.

Normal FT4 Concentration:
Reference FT4 values vary based on age group and pregnancy status, providing a guideline for interpreting FT4 levels in clinical practice.

Age Group/Pregnancy Status Reference FT4 Values (pmol/L):

Age Group/Pregnancy StatusReference FT4 Value (pmol/L)
Less than 4 months11.5 – 28.3
4 months – 1 year11.9 – 25.6
1 year – 7 years12.3 – 22.8
7 – 12 years12.5 – 21.5
12-20 years12.6 – 21.0
Over 20 years10.8 – 22.0
Up to 13 weeks (pregnancy)12.1 – 19.6
13 – 28 weeks (pregnancy)9.6 – 17
28 – 42 weeks (pregnancy)8.4 – 15.6

Analysis Outcome:
Abnormal FT4 levels may indicate underlying conditions such as Graves’ disease, thyroiditis, or thyroid tumors. Further diagnostic tests, including T3 and TSH testing, are recommended to evaluate thyroid function comprehensively.

Regular thyroid screenings, including FT4 testing, are crucial for early detection and management of thyroid disorders. Consulting healthcare professionals for interpretation and follow-up is essential in cases of abnormal FT4 levels.

V.V. Fadeev. “Modern Principles of Diagnosis and Treatment of Hypothyroidism.” ZV. 2010. No. 2.