High thyroid-stimulating hormone (TSH) levels based on a TSH test may mean you have hypothyroidism (under active thyroid), while low TSH levels suggest hyperthyroidism (overactive thyroid).
Nevertheless, there are deviations from these understandings and different perceptions of what a typical TSH score is. This means that the TSH levels may change based on age, whether the person is a man or woman, and other considerations. Eventually, the findings should be considered with other thyroid tests to form an accurate discernment.
What Is TSH?
TSH is a hormone that acts as a messenger to the thyroid gland. When released by the pituitary gland—a small gland located at the base of the brain—TSH stimulates the thyroid to produce more thyroid hormone.
A high TSH suggests your thyroid is under active (hypothyroid) and not doing its job of producing enough thyroid hormone. A low TSH suggests your thyroid is overactive (hyperthyroid) and producing excess thyroid hormone.
How TSH Levels Change
TSH levels seem counterintuitive. Why does a high TSH mean you have an underactive thyroid gland? And why do low levels mean it’s overactive?
To answer that, you have to know that TSH and thyroid hormones are different things. The thyroid hormones are:
- T3 (triiodothyronine)
- T4 (thyroxine)
Your body uses thyroid hormones to regulate all kinds of processes. That includes how fast you burn calories, your heart rate, and many other automatic functions.
When thyroid hormones are low, your pituitary makes more TSH. That signals the thyroid to increase production. When levels are high, the pituitary releases less TSH. That signals the thyroid to slow down. This is why high TSH indicates low thyroid function and vice versa.
Interpreting TSH Levels
Understanding when and why thyroid hormone is released (or not) helps clear up what a low TSH and a high TSH level reveal about your thyroid’s function. TSH is measured in mU/L. This abbreviation stands for milliunits per liter, with one milliunit equating to one-thousandth of a unit.
The normal range for TSH is generally between 0.4 mU/L and 4.0 mU/L. As with most medical conditions and tests, however, there are exceptions to this rule.
Causes of High TSH
A high TSH means different things depending on whether a person has known thyroid disease or not.
In People Without Known Thyroid Disease
A high TSH in people who are not undergoing thyroid disease treatment usually indicates the presence of primary hypothyroidism. This is by far the most common form of hypothyroidism, and it occurs because the thyroid gland produces an inadequate amount of thyroid hormones.
The pituitary gland senses these low levels and increases the production of TSH. An elevated TSH may also occur with normal thyroid function due to the presence of antibodies, proteins made by the immune system.
In People Being Treated for Thyroid Disease
A high TSH may be found in people being treated for either hypo- or hyperthyroidism. With hypothyroidism, a high TSH usually means that the dose of thyroid hormone replacement needs to be increased.
In some cases, however, the dose is okay, but the medication is not being fully absorbed. For example, many foods and medications can affect the absorption of Synthroid (levothyroxine) in your system. Learning what to avoid while taking this medication is important for it to work properly.
With hyperthyroidism, a high TSH usually means that the treatment—be it surgery, radioactive iodine, or medications—was so effective in limiting the production of thyroid hormone that it caused the person to become hypothyroid.
During a normal pregnancy as thyroid hormones are affected by “the pregnancy hormone” human chorionic gonadotropin (hCG).
Less often, a lack of TSH produced by the pituitary gland (due to dysfunction) can lead to low levels of thyroid hormone in the blood—a condition known as central hypothyroidism.
This is an exception to the general rule that hypothyroidism is associated with a high TSH. Central hypothyroidism is uncommon and usually associated with a deficiency of other pituitary hormones. This deficiency leads to a number of other symptoms as well.
In People With Thyroid Disease
In people being treated for hypothyroidism, a low TSH level may mean:
- Overmedication with thyroid hormone replacement
- Interactions that cause increased absorption or activity despite an optimal dose of medication
- Central hypothyroidism
In people being treated for hyperthyroidism, a low TSH level usually means that further treatment is needed to reduce thyroid hormone levels.
It could also mean that a person must continue to be monitored to make sure thyroid hormone levels return to normal. This is often the case for people who have temporary thyroiditis related to pregnancy or chemotherapy treatment.
Tests for Thyroid Cancer
Thyroid cancer may be diagnosed after a person goes to a doctor because of symptoms, or it might be found during a routine physical exam or other tests.
If there is a reason to suspect you might have thyroid cancer, your doctor will use one or more tests to confirm the diagnosis. If cancer is found, other tests might be done to find out more about the cancer.
Medical History And Physical Exam
If you have any signs or symptoms that suggest you might have thyroid cancer, your health care professional will want to know your complete medical history. You will be asked questions about your possible risk factors, symptoms, and any other health problems or concerns.
If someone in your family has had thyroid cancer (especially medullary thyroid cancer) or tumors called pheochromocytomas, it is important to tell your doctor, as you might be at high risk for this disease.
Your doctor will examine you to get more information about possible signs of thyroid cancer and other health problems. During the exam, the doctor will pay special attention to the size and firmness of your thyroid and any enlarged lymph nodes in your neck.
Imaging tests may be done for a number of reasons:
- To help find suspicious areas that might be cancer
- To learn how far cancer may have spread
- To help determine if treatment is working
People who have or may have thyroid cancer will get one or more of these tests.
Ultrasound uses sound waves to create images of parts of your body. You are not exposed to radiation during this test. This test can help determine if a thyroid nodule is solid or filled with fluid. (Solid nodules are more likely to be cancerous.)
It can also be used to check the number and size of thyroid nodules as well as help determine if any nearby lymph nodes are enlarged because the thyroid cancer has spread.
For thyroid nodules that are too small to feel, this test can be used to guide a biopsy needle into the nodule to get a sample. Even when a nodule is large enough to feel, most doctors prefer to use ultrasound to guide the needle.
Radioiodine scans can be used to help determine if someone with a lump in the neck might have thyroid cancer. They are also often used in people who have already been diagnosed with differentiated (papillary, follicular, or Hürthle cell) thyroid cancer to help show if it has spread. Because medullary thyroid cancer cells do not absorb iodine, radioiodine scans are not used for this cancer.
For this test, a small amount of radioactive iodine (called I-131) is swallowed (usually as a pill) or injected into a vein. Over time, the iodine is absorbed by the thyroid gland. A special camera is used several hours later to see where the radioactivity is.
For a thyroid scan, the camera is placed in front of your neck to measure the amount of radiation in the gland. Abnormal areas of the thyroid that have less radioactivity than the surrounding tissue are called cold nodules, and areas that take up more radiation are called hot nodules.
Hot nodules usually are not cancerous, but cold nodules can be benign or cancerous. Because both benign and cancerous nodules can appear cold, this test by itself can’t diagnose thyroid cancer.
Radioactive iodine can also be used to treat differentiated thyroid cancer, but it is given in much higher doses. This type of treatment is described in Radioactive iodine therapy.
If you have been diagnosed with thyroid cancer, a plain x-ray of your chest may be done to see if cancer has spread to your lungs.
Computed Tomography (CT) Scan
The CT scan is an x-ray test that makes detailed cross-sectional images of your body. It can help determine the location and size of thyroid cancers and whether they have spread to nearby areas, although ultrasound is usually the test of choice.
A CT scan can also be used to look for spread into distant organs such as the lungs. One problem using CT scans is that the CT contrast dye contains iodine, which interferes with radioiodine scans. For this reason, many doctors prefer MRI scans for differentiated thyroid cancer.
Magnetic Resonance Imaging (MRI) Scan
MRI scans use magnets instead of radiation to create detailed cross-sectional images of your body. MRI can be used to look for cancer in the thyroid, or cancer that has spread to nearby or distant parts of the body.
But ultrasound is usually the first choice for looking at the thyroid. MRI can provide very detailed images of soft tissues such as the thyroid gland. MRI scans are also very helpful in looking at the brain and spinal cord.
Positron Emission Tomography (PET) Scan
A PET scan can be very useful if your thyroid cancer is one that doesn’t take up radioactive iodine. In this situation, the PET scan may be able to tell whether the cancer has spread.
The actual diagnosis of thyroid cancer is made with a biopsy, in which cells from the suspicious area are removed and looked at in the lab. If your doctor thinks a biopsy is needed, the simplest way to find out if a thyroid lump or nodule is cancerous is with a fine needle aspiration (FNA) of the thyroid nodule. This type of biopsy can sometimes be done in your doctor’s office or clinic.
Before the biopsy, local anesthesia (numbing medicine) may be injected into the skin over the nodule, but in most cases an anesthetic is not needed. Your doctor will place a thin, hollow needle directly into the nodule to aspirate (take out) some cells and a few drops of fluid into a syringe.
The doctor usually repeats this 2 or 3 more times, taking samples from several areas of the nodule. The biopsy samples are then sent to a lab, where they are looked at to see if the cells look cancerous or benign.
Bleeding at the biopsy site is very rare except in people with bleeding disorders. Be sure to tell your doctor if you have problems with bleeding or are taking medicines that could affect bleeding, such as aspirin or blood thinners.
This test is generally done on all thyroid nodules that are big enough to be felt. This means that they are larger than about 1 centimeter (about 1/2 inch) across. Doctors often use ultrasound to see the thyroid during the biopsy, which helps make sure they are getting samples from the right areas.
This is especially helpful for smaller nodules. FNA biopsies can also be used to get samples of swollen lymph nodes in the neck to see if they contain cancer.
When TSH Alone Is Not Enough
During diagnosis, most healthcare providers use the TSH test to evaluate thyroid function and determine the optimal course of treatment. There are times, however, when knowing one’s TSH may be insufficient.
For instance, free T4 in addition to TSH is usually tested if a healthcare provider suspects thyroid dysfunction due to disease of the pituitary gland or hypothalamus. Likewise, if the TSH is normal, but a person still has symptoms of being hyperthyroid or hypothyroid, free T4 may be checked.
TSH is also not necessarily sufficient to monitor hypothyroidism during pregnancy; T4 and free T4 are often recommended. Depending on the clinical situation, other thyroid tests that may be done include (T3), free T3, reverse T3, and thyroid antibody tests.
A TSH level that is optimal for all people cannot quite be established. That’s because TSH levels vary depending on numerous factors, including your age, your thyroid condition, and possibly even how well you sleep at night or when you last ate. Your healthcare provider will consider these factors along with any symptoms you’re experiencing to determine your optimal range.
The TSH test is the gold standard in diagnosing and monitoring thyroid disease. But as with any medical test, there are exceptions to what the test results may mean, as well as situations in which the test may be inaccurate. Your healthcare provider may order additional tests to help determine how your thyroid gland is performing.