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Thyroid Testing | Thyroid Function Test

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Thyroid Testing - General Guidelines

The butterfly-shaped gland at the base of the throat produces important hormones that help regulate metabolism, and many metabolic-related functions. Sometimes however, the thyroid gland becomes enlarged, infected, or diseased causing hormone levels to fluctuate and medical symptoms to occur. Through testing, hormone levels in the blood may be assessed to help determine thyroid function and diagnose thyroid disorders. Testing may also be ordered to detect pituitary dysfunction as well. Pregnancy can affect thyroid test results, as well as certain medications including estrogen, such as birth control pills.

Why Test Is Used

Thyroid testing may be ordered for a number of medical reasons relating to either the overproduction, or the underproduction of thyroid hormones.

Blood test results may help determine or detect hyperthyroidism, which results in the overproduction of thyroid hormones.

Some conditions that can cause hyperthyroidism include:

  • Graves' disease—A malfunction in the immune system causes the overproduction of thyroid hormone.
  • Toxic adenomas—Nodules grow within the thyroid, secreting hormones.
  • Subacute thyroiditis—Thyroid inflammation causes “temporary” leakage of hormones and hyperthyroidism.
  • Malfunctioning pituitary gland/thyroid cancer—Can lead to hyperthyroidism.

Hypothyroidism is caused by the production of too little thyroid hormone and leads to low energy levels in individuals.

Conditions that promote hypothyroidism include:

  • Hashimoto's thyroiditis—This autoimmune disorder attacks and kills thyroid tissue, stopping hormone production altogether.
  • Thyroid gland removal—Sometimes the thyroid gland must be surgically removed, or chemically destroyed, halting hormone production.
  • High iodine exposure—Certain contrast dyes used in X-rays, cold and sinus medicines, and the heart medication, amiodarone may put individuals at risk for hypothyroidism, (especially if there is a history of thyroid problems).
    Lithium—Use of this medication may lead to hypothyroidism.

Hypothyroidism is especially critical in young infants, and can cause dwarfism and mental disabilities. Left untreated at any age, hypothyroidism can also lead to a rare, but fatal, myxedema coma, requiring immediate hormone treatment.

How does it work?

The thyroid gland works to produce a series of hormones that signal, support, or regulate nearly every metabolic function within the body. When thyroid levels are outside normal ranges, it could be an indication of either, a thyroid disorder, or a problem with the pituitary gland that works in tandem with the thyroid, much like a heater and a thermostat.

In a functioning thyroid the primary thyroid hormone, thyroxine, also referred to as, “T4” is secreted by the thyroid gland and converted to triiodothyronine, (T3) (to become effective), in the liver and the brain. Thyroid Stimulating Hormone (TSH), produced in the pituitary gland at the base of the brain, controls the amount of T4 produced in the thyroid. Conversely, the amount of TSH the pituitary gland sends into the bloodstream depends on the amount of T4 the pituitary “sees”. Together, T3 and T4 are produced and released based on levels of each other. Blood tests for thyroid function may measure TSH, T4, T3, and Free T4 (not bound to transport proteins).

About Results

Thyroid test results may differ between laboratories. Any indication above or below normal ranges could indicate one of the conditions referred to above.


Serum thyroxine (T4)4.6-12 ug/dl

Free thyroxine fraction (FT4F) Normal Ranges = 0.03-0.005%

Free Thyroxine (FT4) Normal Ranges = 0.7-1.9 ng/dl

Thyroid hormone binding ratio (THBR) Normal Ranges = 0.9-1.1

Free Thyroxine index (FT4I) Normal Ranges = 4-11

Serum Triiodothyronine (T3) Normal Ranges = 80-180 ng/dl

Free Triiodothyronine l (FT3) Normal Ranges = 230-619 pg/d

Free T3 Index (FT3I) Normal Ranges = 80-180

Radioactive iodine uptake (RAIU) Normal Ranges = 10-30%

Serum thyrotropin (TSH) Normal Ranges = 0.5-6 uU/ml

Thyroxine-binding globulin (TBG) Normal Ranges = 12-20 ug/dl T4 +1.8 ugm

TRH stimulation test Peak (TSH) Normal Ranges = 9-30 uIU/ml at 20-30 min

Serum thyroglobulin l (Tg) Normal Ranges = 0-30 ng/m

Thyroid microsomal antibody (TMAb) Normal Ranges = Varies with method

Thyroglobulin antibody (TgAb) Normal Ranges = Varies with method


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