Understanding thyroid tests

In this examination, the doctor carefully feels your neck where your thyroid gland is, usually from behind you. She or he will pay attention to what the thyroid gland feels like and whether it is bigger than usual.

An enlarged thyroid – also called a goiter – can be a sign of an iodine deficiency that hasn't yet affected the function of the thyroid. But it could also be a sign of an overactive thyroid (hyperthyroidism) or an underactive thyroid (hypothyroidism), where too many or too few thyroid hormones are made respectively.

Nodules that can be felt from the outside may also be a sign of a thyroid problem. But sometimes people have an enlarged thyroid gland or nodules without it affecting the function of their thyroid gland.

So this palpation examination can only tell us whether there might be a thyroid problem. Further tests and examinations are needed in order to be sure.

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 made by the thyroid gland. 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. Most thyroid hormones are bound to certain proteins in the blood. Only unbound “free” thyroid hormones are active and have an effect, though. So only the free thyroid hormones are measured (FT3 and FT4 – where “F” stands for “free”).

When trying to find out what is causing a thyroid problem, a blood test is done to look for thyroid antibodies. These antibodies are made if the immune system attacks the body’s own thyroid tissue by mistake. They may also block the effect of thyroid hormones.

Another hormone produced by the thyroid gland is 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.

If the levels of T3 and T4 in the blood are too high or too low, there is an imbalance between the amount of thyroid hormones needed by the body and the amount of thyroid hormones available. The levels of the following substances can help to find out what is causing the imbalance:

• Thyroid-stimulating hormone (TSH): High TSH levels are a sign of an underactive thyroid (hypothyroidism). The pituitary gland produces more TSH in order 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 may be a sign of an overactive thyroid, and low levels could be a sign of an underactive thyroid.
• 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 taking 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 spatial image that is shown on a screen.

The ultrasound image shows whether the thyroid gland is bigger than normal (enlarged). An enlarged thyroid gland could be a sign that it is underactive or overactive. To know for sure whether the thyroid gland really is making too many hormones or not enough, a blood test has 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 – in some cases – 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). 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. More active areas of the thyroid gland absorb more of the substance, and less active areas absorb less. The thyroid scan image shows how much of the radioactive substance has been absorbed in different areas of the thyroid gland. 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 medication you are taking.

Thyroid scans can have adverse effects: In rare cases, the needle might damage blood vessels or nerves, or the skin at the site of injection may become infected. 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.

Thyroid scans allow us to look at how active the thyroid tissue is: The more active it is, the more blood goes through it and the more of the injected substance builds up in it. Active areas of the thyroid gland can be clearly seen on the thyroid scan image. Depending on how much of the injected substance builds up, “cold” nodules and “hot” nodules can be identified: Cold nodules have less of the radioactive substance in them because the metabolism in their tissue is slower (less active). In most cases, cold nodules are caused by harmless changes in the tissue. In very rare cases, though, they are caused by a thyroid tumor.

Hot nodules have more of the radioactive substance in them – their metabolism is faster (more active) and their tissue produces more hormones. If hot nodules grow bigger than a certain size, they may lead to an overactive thyroid.

In fine needle aspiration, a very fine, hollow needle is inserted into the thyroid gland to remove tissue or fluid. It’s usually not necessary to numb the area with an anesthetic because it’s not more unpleasant than having a normal blood sample taken from an arm. To be able to see the procedure better, doctors often use an ultrasound machine too.

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.

Fine needle aspiration can provide more detailed information about what is causing the changes in thyroid tissue seen in an ultrasound or thyroid scan. The possible causes include an inflammation or a benign (non-cancerous) or malignant (cancerous) tumor. Tissue samples are taken and then sent to a laboratory, where the cells are examined under a microscope. Fine needle aspiration can be used to empty out fluid-filled cysts too.