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Thyroid Testing

Thyroid Testing

Thyroid Testing

by Rashmi Gulati, MD
Rashmi Gulati Head Shot

What is TSH Level

TSH is know as "Thyroid stimulating hormone". TSH a blood test used to check for thyroid gland problems. TSH is produced when the hypothalamus releases a substance called TRH, or "thyrotropin-releasing hormone". TRH triggers the pituitary gland to release TSH. The thyroid gland is stimulated by TSH to make triiodothyronine (T3) and thyroxine (T4), two hormones. T3 and T4 help control the body's metabolism.

Thyroid dysfunction is a common occurrence that frequently goes undetected. Symptoms of thyroid disorders often develop in an insidious fashion and, to confuse the picture further, overlap with numerous other health concerns, which can make detection and accurate diagnosis of thyroid disorders challenging. In many cases the patient's clinical features and thyroid function tests require careful consideration.

Routine blood tests for thyroid function measure circulating levels of TSH, T4, and T3 in the bloodstream. Because thyroid hormones do not operate by themselves within the bloodstream, these values have their limitations. Thyroid hormones use receptors on cells throughout the body, to turn on and off a multitude of processes within tissues. Blood tests measure circulating levels of thyroid hormones within the blood stream, not what is taking place on the cellular level.

Comprehensive thyroid evaluation at Patients Medical can detect alterations in thyroid function that may be missed on more routine blood test screening. Given proper testing, evaluation, and treatment, adverse health conditions associated with thyroid dysfunction can, for the most part, be detected and avoided.

TSH assay and circulating T4: Conventional thyroid tests generally include a TSH assay and measurement of total T4 (inactive thyroid hormone) in the blood, providing you with a snapshot of your circulating TSH and T4 levels at that moment in time. These standard tests are often adequate to the task of detecting the most common thyroid disorders and those of an overt nature, such as primary hypothyroidism.

Testing for Primary, Secondary, Tertiary, and Central Hypothyroidism—Some Definitions

In primary hypothyroidism, thyroid hormone deficiency is due to a defect in the thyroid gland.

In secondary hypothyroidism, the hypothalamus produces insufficient thyrotropin-releasing hormone (TRH), or the pituitary produces insufficient thyroid-stimulating hormone (TSH).

Tertiary hypothyroidism is another term for secondary hypothyroidism where deficient TSH secretion by the pituitary is due to deficient TRH secretion by the hypothalamus.

Central hypothyroidism is another term used in place of either secondary or tertiary hypothyroidism, where thyroid hormone deficiency is due to a problem with the hypothalamus, or with the pituitary gland, or with hypothalamic–pituitary portal circulation.

While the overwhelming majority of patients with hypothyroidism have primary hypothyroidism (see box of definitions, at right), in many others the picture is not so clear-cut. Screening for subtle forms of thyroid hormonal dysregulation, such as subclinical hypothyroidism, or less straightforward forms such as central hypothyroidism or thyroid hormone resistance, nearly always involve further testing and review. Making a differential diagnosis between types of thyroid disorders will often require the clinician to pursue additional testing, as does determining the extent of a patient's thyroid disease.

Complete thyroid panel: Complete thyroid panel testing evaluates TSH, T4, free T4, T3, and free T3 levels, as well as antithyroid antibodies.

Reverse T3: Reverse triiodothyronine, also referred to as reverse T3 (rT3), is the molecule formed when the active T4 hormone is deiodinized. Increased levels may be indicative of thyroid imbalance or dysfunction. In patients experiencing symptoms of poor energy levels, poor ability to regulate body temperature, and other generalized symptoms of functional hypothyroidism, there may be underproduction of T3 or overproduction of rT3. These are both found frequently where the HPT axis is overactivated.

Antithyroid antibodies: Testing for the presence of antithyroid autoantibodies, such as anti-thyroglobulin antibodies (TgAb), anti-peroxidase antibodies (TPOAb), and anti-microsomial antibodies (TMAb), can help to pinpoint an autoimmune component to a thyroid disorder. In particular, Hashimoto's thyroiditis as a cause of hypothyroidism can be definitively diagnosed on the basis of antibody levels. Well before an individual develops Hashimoto's, however, the presence of even marginally elevated antithyroid antibodies can impair proper thyroid hormone metabolism and sensitivity.

Mineral and vitamin deficiencies: Testing for iodine and selenium sufficiency in the body may also be warranted, as adequate levels of these minerals are instrumental to synthesis and conversion of thyroid hormones, respectively. Testing for adequate zinc levels in the body may be ordered, as research suggests zinc influences the metabolism of thyroid hormones. Vitamin D is one nutrient that is crucial for establishing immune balance, and preventing the production of autoantibodies, and suppressing the development of autoimmune diseases in general. Overload with toxic minerals, like bromine, chloride, and mercury also interfere with iodine and thyroid hormone production. 

TRH Stimulation Test: The TRH-stimulation test is a challenge test that was more widely used in the past, before the development of newer-generation radioimmunoassay tests for TSH and free thyroid hormones. However, the TRH stimulation test remains especially useful as a means to evaluate function along the hypothalamic–pituitary–thyroid (HPT) axis, for determining the underlying cause of central hypothyroidism.

To understand how the TRH stimulation test works, it is helpful to quickly review the interaction of brain hormones with the thyroid along this neuroendocrine axis.

  • Your hypothalamus is a gland in your brain that secretes thyrotropin-releasing hormone (TRH).
  • TRH is released into the hypothalamic–pituitary portal blood vessels, which transport it to the pituitary gland—also in the brain.
  • TRH regulates the pituitary synthesis and release of thyrotropin, also known as thyroid-stimulating hormone (TSH).
  • TSH tells the thyroid to capture iodine from the blood for synthesizing, storing, and releasing thyroid hormones, primarily in the form of T4, but also as T3.

Most practitioners will first order a baseline TSH test (also called basal serum TSH or TSH assay), to measure circulating level of TSH in the bloodstream at that point in time. The TSH test results are then interpreted, with levels higher or lower than “normal” range considered evidence of potential thyroid disease.

However, it has been noted that diagnosing patients with central hypothyroidism is often complicated because they can present with widely varying clinical symptoms and hormonal profiles. Patients with pituitary or hypothalamic disorders causing secondary hypothyroidism can have basal serum TSH levels that are either low, or normal, or even in the high-normal range. Alternatively, central hypothyroidism can be accompanied by normal TSH values and thyroxine (T4) or free T4 (fT4) levels in the lower part of normal range. There are cases where central hypothyroidism simply cannot be detected simply by measuring peripheral thyroid hormones and TSH levels. In addition, other pituitary hormone deficiencies seen in association with central hypothyroidism may mask the usual clinical manifestations of thyroid hormones insufficiencies. These more nuanced forms of neuroendocrine dysfunction are where the TRH stimulation test can be invaluable.

The TRH test uses a synthetic preparation of TRH known generically as protirelin (pronounced proe-TYE-re-lin), given to stimulate the pituitary to release TSH. A second blood sample is drawn 20–30 minutes later, and the TSH level is retested.

By challenging the thyroid, the TRH-stimulation test evaluates the thyroid's ability to function in real time. The difference between the TRH-stimulation test and a simple TSH test is akin to comparing a cardiac stress test to a cardiogram, or a glucose tolerance test to fasting glucose level. Each provides another piece to the patient's total clinical picture. In a stimulation test, the challenge may reveal impairment in thyroid function, or it may disclose that the impairment lies elsewhere along this important neuroendocrine axis.

Thyroid Scanning

In certain cases, imaging studies of the thyroid can be useful for evaluating anatomical or functional abnormalities. A thyroid scan can be done to check for thyroid nodules, for example, or performed in conjunction with a radioactive iodine uptake test (RAIU) to determine whether the thyroid gland is working properly.

In thyroid scan with RAIU, a radioactive iodine tracer is given in advance of the imaging study, in drink or pill form. If an individual is allergic to iodine, a technetium tracer can be used. Over a period of about four hours, the iodine tracer is taken up by the thyroid tissues, at which point the individual lies prone below a specialized camera designed to detect molecules of the radioactive material. While the patient lies still over a period of 30–60 minutes, the technician captures a series of images of the thyroid gland. The patient will usually be asked to return a day later, to have a second set of images taken, which requires only a few minutes.

These images can be contrasted and compared to detect patterns of uptake that provide information about the functioning of the tissues. When certain regions acquire greater amounts of the tracer, this can be indicative of certain forms of thyroid disease. This information can be helpful in guiding decisions on how best to treat the problem. For example, the thyroid scan may reveal an area that is suspicious for thyroid cancer. Additional scanning with ultrasound can help determine the extent of the lesion and whether surgery may be required. Alternatively, scanning may demonstrate that the thyroid gland is overactive, or help to predict its response to medications.

Whole Body Scan: If metastatic thyroid cancer is suspected, a whole-body scan can be performed to determine whether the cancer has spread to other regions.

Testing for Thyroid Disease and Disorders—Citations and Further Reading


Date of Publication: 09/05/2005
Article Last Updated: 01/23/2014

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