What Is A Thyroid Function Test?
Your thyroid gland is an important part of our metabolism, and our body's functioning.
The thyroid gland controls metabolism, or energy derived from the foods that you
eat, and maintains your normal body temperature, and heart rate. Your appetite and
your digestive system will also be affected by your thyroid gland.
The thyroid gland is also important in muscle and bone development, as well as normal
brain and nervous system functioning. The thyroid gland is located in the
middle of your neck.
There are many thyroid hormones. Each has a different role. It all starts with iodide,
a salt that is now found in most brands of table salt.
The iodide that you ingest is taken in by the thyroid gland. Here begins a complex
- Once converted to iodine, your serum T4, (called thyroxine) hormone is formed.
- The thyroxine hormone is converted into a more potent or active form of hormone,
your serum (blood) T3. This is also called triiodothyronine. These T4 and T3 hormones
will circulate in your body, and regulate your metabolism.
- Your FT4I (free thyroxine index) is another test used to measure your thyroid function.
- All of these thyroid function tests are all indicators
of your thyroid gland function.
- Thyroid stimulating hormone (TSH), is the most common test that is ordered if your
healthcare provider thinks that your thyroid gland is not functioning properly.
A gland, called the pituitary gland, secretes TSH. TSH is responsible for taking
iodide out of your blood stream, and into your thyroid gland. TSH is also responsible
for production of the thyroid hormone.
- If you have an abnormally high or low TSH level, whether
it is elevated or decreased, your healthcare provider may order serum (blood) T3, serum T4, or FT4I tests. While there are many other tests that can be performed,
these are the most common tests for diagnosing hyperthyroid or hypothyroid,
and the most specific indicators as to how well your thyroid gland is functioning.
- Based on your laboratory values of your thyroid function tests,
and your symptoms, your healthcare provider may decide whether or not to treat your
disease, whether it be hyperthyroidism or hypothyroidism.
- In cases of hyperthyroidism, your TSH level will be decreased, because there is
too much thyroid hormone in your blood. Your TSH levels will be decreased, if:
- You have Grave's hyperthyroidism. This is the most common cause of hyperthyroidism.
For many reasons, your body is producing too much of the thyroid hormone.
- Your thyroid gland is too large, or you may have a thyroid tumor producing too much
of the thyroid hormone
- Your thyroid is infected, or inflamed, producing too much thyroid hormone
(resulting in hyperthyroidism).
- Your thyroid gland was injured, or removed, and you are taking too much thyroid
hormone in a pill form, as a replacement.
- In cases of hypothyroidism, the thyroid function test - TSH will be
increased, because there is not enough thyroid hormone in your blood.
Your TSH levels will be increased, if:
- Your thyroid gland is not working as it normally should
- Your thyroid gland is infected or inflamed, as in Hashimoto's thyroiditis, or autoimmune
thyroiditis. This occurs when your body is attacking your thyroid gland, for some
unknown reason. Thyroiditis can also be seen after pregnancy.
- Your overactive thyroid gland was removed, and you are not taking enough hormone
pills to replace the normal thyroid hormone that was lost, thus resulting in high
- You may have a damaged thyroid gland.
- Too much or too little iodide intake- Either you are not taking enough iodide in
your diet, or you are taking too much iodide in your diet. Too much iodide will
suppress the hormone production.
- Certain medications, such as lithium (for a psychological condition, called bipolar
disorder), or amiodarone (for heart rhythm abnormalities), may cause low TSH levels
or hypothyroidism. Thalidomide, a drug that is used in many forms of cancers because it prevents new blood cell formation to tumor cells (anti-angiogenesis), may
cause hypothyroidism with long-term use.
- Radiation to the neck area may cause hypothyroidism (as in head and neck cancer)
Normal levels of thyroid hormones in thyroid function tests:
Normal Levels of Thyroid Hormones*
*normal values may vary from laboratory
Symptoms of Hypothyroidism and Hyperthyroidism:
- If you have a hyperactive (over active) thyroid, you may notice these symptoms:
- Weight loss, eye or vision changes.
- Palpitations, rapid heartbeat, shakiness, sweating, feeling "hot"
- Diarrhea, stomach and bowel abnormalities or anxiety
- Insomnia (trouble sleeping), fatigue, weakness, or hair loss
- An enlarged thyroid gland, or a goiter, may result
- If you have a hypoactive (under active) thyroid, you may notice these symptoms:
- Weight gain, depression, forgetfulness, decreased concentration or fatigue
- Hoarseness, feeling cold or sluggish, hair loss, dry skin, round puffy face (seen
with a severely depressed thyroid gland), or tingling in your hands/feet
- High cholesterol, constipation, irregular or heavy menstrual periods in pre-menopausal
women, low sex drive or infertility.
Things You Can Do About Thyroid Malfunction:
If you notice any of these symptoms, visit your healthcare provider for an examination.
The diagnosis of hypothyroid and hyperthyroid include a history (your family and
health history may place you at risk), physical examination, and key
thyroid function tests. Sometimes, an ultrasound or the
thyroid gland may also be ordered. There are treatments for each of these diseases.
Follow your entire healthcare provider's instructions regarding laboratory testing
for your disease, and follow up care depending on the results. .
- Try to exercise. Make a daily walk alone, or with a friend or family member a part
of your routine. Even light walking or aerobic activity may help you to promote
the flow of oxygen in your lungs and blood (oxygenation), and make you feel better.
- Follow a good diet. In general, increase your intake of fresh fruits and vegetables.
Limit your intake of fats. Eat carbohydrates (such as sugars and pastas) in moderation.
- Make sure you tell your doctor, as well as all healthcare providers, about any other
medications you are taking (including over-the-counter, vitamins, or herbal remedies).
These can cause interactions with other medications.
- Remind your doctor or healthcare provider if you have a history of diabetes, liver,
kidney, or heart disease.
- Keep yourself well hydrated. Drink two to three quarts of fluid every 24 hours,
unless you are instructed otherwise.
- If you experience symptoms or side effects of your disease or therapy, especially
if severe, be sure to discuss them with your health care team. They can prescribe
medications and/or offer other suggestions that are effective in managing such problems.
- Keep all your appointments.
Drugs That May Be Prescribed by Your Doctor:
If you have an overactive thyroid (hyperthyroidism) your doctor or healthcare provider
- Drugs to decrease your thyroid levels - may include Methimazole (Tapazole®), or propylthiouracil (PTU). These pills are used once a
day, to decrease your thyroid hormone production. You may need to take this once
a day, at the same time each day, for a year or longer, to treat your hyperthyroidism.
- Propranolol - this is a medication to treat your hyperthyroidism. Also used to treat
heart problems, including palpitations, it will decrease your resting heart rate.
- If you are on any of these medications, you must follow your healthcare provider's
recommendations, including medication schedule, and follow up visits.
If you have an underactive thyroid (hypothyroidism) your doctor or healthcare provider
- Levothyroxine: This is a synthetic form of a thyroid hormone that will normalize
all of your thyroid functioning. It is given in a pill form. You may not see a change
in your symptoms for 3 to 6 weeks. You may take this pill once a day, with or without
- You will be required to have periodic thyroid function tests particularly
checking your TSH levels to monitor the effectiveness of your dose.
Based on your condition, and when you were diagnosed, your healthcare provider will
arrange a schedule that is right for you.
- There are many drug interactions with the thyroid hormone. Make sure your tell all
your healthcare providers what medications you are taking, so that your dose can
be maintained at a "therapeutic" level.
Return to list of Blood
Note: We strongly encourage you to talk with your health care professional
about your specific medical condition and treatments. The information contained
in this website is meant to be helpful and educational, but is not a substitute
for medical advice.