Thyroid Hormone Levels: T4, T3, T2, and T1

Your thyroid gland, which is situated in the lower front part of your neck, produces hormones that play a role in a variety of functions throughout the body, and impact many other hormones outside the thyroid. The two main states of dysfunction seen in thyroid conditions are called hyperthyroidism (too much hormone) and hypothyroidism (too little), and can be induced by a number of different individual thyroid disorders. There are many conditions involving this gland, each of which presents a unique pathology and characteristic clinical picture, usually seen when lab values for these hormones fall outside their normal ranges.

What hormones are made by the thyroid gland?

Of the several “thyroid hormones” produced by this gland, there are two that are clinically relevant. One is called thyroxine, also known as T4, while the other is named triiodothyronine, commonly referred to as T3. They are both heavily involved in energy management and a variety of biochemical and metabolic reactions and functions throughout the body. While T4 is more abundant in the bloodstream, it is T3 (which is derived from the conversion of T4 in the kidneys and liver) that possesses the most potency and is thus responsible for most metabolic activity. In the event that a patient requires supplementation because of low hormone levels, both forms are available therapeutically, and an endocrinologist will be able to explain which is needed and why.

What are the optimal levels of the main thyroid hormones?

The normal lab range of total T4 in adults is approximately 5-14 micrograms per deciliter (mcg/dL), while free (not bound to proteins) T4 is considered normal between 0.8–2.0 nanograms per deciliter (ng/dL). Total T3 should be between 80-200 ng/dL, and free T3 should remain within a range of 2.3–4.2 picograms per deciliter (pg/dL). However, optimal levels of any hormone are often debated and may differ from country to country, doctor to doctor, or institutionally.

What is the difference between “free” T3 and T4 and “total” T3 and T4?

Total T4 and T3 indicate the total amounts of these hormones produced by the thyroid gland, whereas free or “unbound” T4 and T3 measure the amounts of hormones that are bioactive, or actually available to your cells and tissues (typically less than 1% compared to the total produced). Looking at the free hormones usually gives a better clinical picture to the doctor, and is generally more helpful for diagnostic and treatment purposes.

What is TSH or thyroid-stimulating hormone?

TSH, also known as thyrotropin, is actually not a thyroid hormone; rather, it’s a pituitary hormone that stimulates the production of thyroid hormones. High levels of TSH may indicate that the body is hypothyroid, though there are other reasons for elevations, such as a primary pituitary tumor. The traditional reference range for TSH has been between 0.5 and 6.0 milli-international units per liter (mIU/L), although many endocrinologists now recommend that it remain between 0.3 and 3.0 mIU/L. However, if the upper portion of the normal range were lowered to just 3.0, approximately 20% of the population would be considered hypothyroid, instead of about 5% now. TSH levels outside the normal range may be associated with a number of disease states.

What are T2, T1 and calcitonin and why doesn’t my doctor measure those?

T2 and T1 are thought to play minor roles in the thyroid and rest of the body, primarily serving as precursors to and byproducts of T4/T3 formation. They may be more involved in thyroid function, but as far as we can tell right now, they have little impact beyond that noted. Calcitonin is a thyroid hormone that, along with parathyroid hormone (PTH), regulates calcium, and it is another measure that can be too low with hypothyroidism.

T2 and T1 aren’t measured because they have no demonstrated clinical value. Calcitonin is sometimes measured as part of an “extended thyroid panel”, when trying to settle on a definitive diagnosis where none is clear, especially in the cases of some suspected endocrine tumors. Values for the latter are considered normal below 10 pg/mL in men and 5 pg/mL in women. Unlike the standard medical treatments for hypothyroidism, desiccated thyroid supplements often contain T1, T2, T3, T4 and calcitonin.

Questions for your doctor:

• Why do different doctors use different measurements for the “normal” ranges of thyroid hormones?
• What are the benefits and are there any risks of taking desiccated thyroid products?
• Is a TSH test diagnostic of hypothyroidism by itself?

Reference
http://www.ncbi.nlm.nih.gov/pubmed/18753096

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