T4 helps to confirm a diagnosis of hyper or hypothyroidism. It provides valuable information.

In this video, we will be talking about the thyroid hormone known by the names: thyroxine, tetraiodothyronine, or for short it is just called T4. T4 is the main hormone produced by the thyroid gland. The 4 in the name refers to the number of iodine molecules.

T4, it is not as bioavailable (or usable) to the cells as T3 is. Think of T4 as a way of storing thyroid hormone like you may store food in a can in your kitchen for a later date. T4 is more stable version of thyroid hormone than T3. It lasts longer for future use, typically 4 to 5 weeks. T4 can be measured in the blood two ways:

Measured T4 Blood tests & ranges (lab references vary depending on lab):

• Total T4 - This is the total of the bound T4 (most circulating T4 is bound to certain plasma proteins) PLUS the free T4.
  • Typical lab range for Total T4 is referenced as: 5-12 mcg/dL
  • Optimal range for Total T4: 10-12 mcg/dL
• Free T4 - Approximately 0.02% of T4 circulating in the blood is unbound or “free,” meaning unattached to a protein.
  • Typical range for Free T4 is referenced as:  0.7-1.8 ng/dL
  • Optimal range for Free T4: 1-1.5 ng/dL

High T4 Levels: This confirms an overly active thyroid gland resulting in overproduction of thyroid hormones. This can be due to conditions like:

• Autoimmune disease called Graves' disease
• Thyroid nodules
• Thyroiditis.

Symptoms of hyperthyroidism may include:

• Weight loss
• Heat intolerance
• Increased appetite
• Nervousness
• Heart palpitations.

Low T4 Levels: Confirms an underactive thyroid gland resulting in insufficient production of thyroid hormones. This can result from conditions like:

• Autoimmune disease called Hashimoto's thyroiditis
• Iodine deficiency
• Surgery
• Certain medications

Symptoms of hypothyroidism may include:

• Fatigue
• Weight gain
• Cold intolerance
• Constipation
• Depression.

Refer back to the extensive list of symptoms in this document hand out here: Common Signs and Symptoms of Hyper and Hypothyroidism

Factors Influencing Total T4 Levels

• Protein Levels: Since most T4 is bound to proteins, changes in protein levels can affect total T4 levels, can be seen during pregnancy, liver disease, or nephrotic syndrome (kidney disease).
• Medications: Certain medications can influence total T4 levels. Examples:
  • Estrogen (like hormone contraception or hormone therapy) can increase thyroid-binding proteins, which can increase total T4 levels, with normal free T4.
  • Glucocorticoids (like prednisone or other steroids) and androgens (like testosterone or DHEA) can decrease them.

For more information about how medications can affect thyroid function, click here.

I hope the TSH and T4 labs are starting to bring some things together for you in your understanding of how to interpret thyroid labs.

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