Understanding thyroid tests

In this examination, the doctor carefully feels your neck where your thyroid gland is, usually from behind you. It is easier to feel the thyroid from this position. They will pay attention to what the thyroid gland feels like and whether it is bigger than usual.

An enlarged thyroid is also called a goiter. The most common cause of a goiter is iodine deficiency. The thyroid gland often continues to work normally.

But a thyroid can also become enlarged for other reasons. It may be that many benign (non-cancerous) nodules develop in it, or it could be caused by the thyroid being overactive (hyperthyroidism) or underactive (hypothyroidism).

So doctors will only be able to tell so much after feeling the thyroid gland. Further testing is needed in order to find out what the cause is any whether there could be a problem with the functioning of the gland.

The thyroid gland constantly releases a certain amount of hormones into the blood. So a blood test can be done to determine the amounts of hormones it is making.

Blood tests can measure the levels of TSH and the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Abnormal TSH levels may be an early sign of thyroid problems. For this reason, it is common to only measure the TSH level at first. If the TSH level in the blood is higher or lower than normal, the levels of the thyroid hormones T3 and T4 are also measured.

If doctors think that an autoimmune reaction is causing problems in the functioning of the thyroid, they will look at the levels of thyroid antibodies. These are antibodies made by the immune system that bind to the thyroid tissue. This can cause inflammation in the thyroid gland, stop it from functioning or simulate it to make too many hormones.

Some parts of the thyroid make another hormone called calcitonin. The level of calcitonin in the blood is usually only measured if there is reason to believe that someone has a certain type of thyroid cancer that increases the amount of calcitonin.

The levels of the following substances can help to find out what is causing problems in the production of the thyroid hormones:

• Thyroid-stimulating hormone (TSH): High TSH levels are a sign of an underactive thyroid (hypothyroidism). They show that the pituitary gland is trying to stimulate the thyroid gland to produce thyroid hormones. Very low TSH levels in the blood may be a sign of an overactive thyroid (hyperthyroidism). The pituitary gland then produces less TSH, in order to stop “telling” the thyroid gland to make more hormones.
• Free triiodothyronine (FT3) and free thyroxine (FT4): High levels of free thyroid hormones in the blood can indicate a hyperfunction. If they are low, this can be a sign of a thyroid hypofunction.
• Thyroid antibodies: The concentration of thyroid antibodies in the blood is higher in certain disorders where the body’s immune system attacks the thyroid gland. These include Hashimoto’s disease and Graves’ disease.
• Calcitonin: Calcitonin levels are usually higher in a certain type of thyroid cancer. But high levels of calcitonin can also be a sign of other diseases, such as kidney failure. Calcitonin levels play an important role in calcium and bone metabolism too.

The concentration of TSH and thyroid hormones in the blood can also be influenced by the long-term use of certain medications. These medications include:

• Acetylsalicylic acid (the drug in medicines like Aspirin)
• Steroids
• St. John's wort
• Certain diuretic medications containing furosemide
• Thyroid medications

Because of this, it’s important to let your doctor know about any medications you are using before having a blood test.

In a thyroid ultrasound (sonography), sound waves are sent to the thyroid gland. Depending on the type of tissue they bounce off there, the sound waves are then sent back with different intensities or not at all. To do the examination, a small amount of gel is put on the “head” (transducer) of the ultrasound device, which is then moved over the neck. The sound waves that bounce off the thyroid tissue are measured by the transducer and turned into a two-dimensional or spatial image that is shown on a screen.

The ultrasound image shows the doctor whether the thyroid gland is bigger than normal (enlarged). It can't show whether the enlarged gland is making too many or two few hormones. That is why a blood test typically needs to be done too.

Changes in the tissue – like nodules or cysts – can also be seen in ultrasound images. If nodules (lumps) are discovered, other examinations may be done to find out more about them. These include a thyroid scan or magnetic resonance imaging (MRI).

A thyroid scan (thyroid scintigraphy) is done to see how active the thyroid gland is (how many hormones it is producing in which parts of the thyroid). Before doing the scan, a slightly radioactive substance is injected into an arm vein. This substance travels around the body in the bloodstream, but most of it is absorbed by the thyroid gland. The more active the individual areas of the thyroid gland are working, the more of the substance they absorb. The thyroid scan image shows how much of the radioactive substance has been absorbed in which area. This image is called a scintigram.

It may be necessary to stop taking certain drugs, such as thyroid medications, before having a thyroid scan. Because of this, it’s important to let your doctor know about any medications you are taking before the scan.

In rare cases, the needle might damage blood vessels or nerves, or the skin at the site of injection may become infected during a thyroid scan. Allergic reactions, particularly to the injected substance, are possible.

People are exposed to a small amount of radiation during the scan. Only very small amounts of radioactive substances are used, though, and they are broken down in the body within a few days without causing any harm.

Thyroid scans allow us to look at how evenly the thyroid gland is working or whether certain areas are especially active. Those are referred to as “hot” nodules. Hot nodules are benign (non-cancerous), but they could be the cause of an overactive thyroid. Areas that produce fewer hormones compared to the rest of the thyroid, or none at all, are referred to as "cold." nodules. Most cold nodules are also benign. In very cases, though, they are caused by a malignant (cancerous) thyroid tumor.

A technique called fine needle aspiration is typically used to take a sample of the thyroid tissue. The doctor inserts a very fine, hollow needle into the part of the thyroid gland that needs to be tested in order to remove tissue or fluid.

It is usually not necessary to numb the area with an anesthetic because it’s not more unpleasant than having a blood sample taken from your arm.

In rare cases, the place where the needle was inserted may become bruised or infected. If you take medication that stops blood from clotting (anticoagulants), it’s important to talk to your doctor about whether you should stop taking it before the procedure. Sometimes the aspiration needs to be repeated or the doctor needs to use a larger needle.

After the tissue sample has been examined in a laboratory, doctors can see whether the thyroid tissue is inflamed or has undergone malignant changes.