The thyroid gland makes hormones that control energy use and metabolism throughout the whole body. When levels of these hormones are too low or too high, a variety of symptoms may result.
What constitutes thyroid levels that are too low or too high? How do you know whether you have proper thyroid levels? What should your thyroid levels be?
What are thyroid levels and how are they tested?
To check the function of the thyroid, blood tests are needed. Some of the tests that may be used include:
- Thyroid stimulating hormone (TSH). This is a hormone that’s made by the pituitary gland, and it stimulates the thyroid gland to make T4 and T3.
- Free T4 and T3. T4 and T3 are the thyroid hormones themselves. They’re considered “free” when they’re not bound to thyroid binding globulin (TBG), which is a carrier protein that transports the thyroid hormones in the blood.
- Antithyroid antibodies. In Hashimoto’s disease, the body mistakenly makes antibodies against the thyroid, and these interfere with its function. Hashimoto’s disease is the most common cause of hypothyroidism (low thyroid hormone levels).
- Thyroid stimulating antibodies (TSI). In Graves’ disease, there are also mistaken antibodies, but these bind to the thyroid and stimulate it to make extra thyroid hormones. This is a common cause of hyperthyroidism (high thyroid hormone levels).
It’s important to note that the normal levels for each test may be somewhat different in different laboratories. This is because of differences in the specifics of the techniques used for testing. Each laboratory will report the reference range for TSH, free T4, and other thyroid tests. This is the range of values that’s considered normal on the tests performed by that specific laboratory.
When you have a thyroid test, then the laboratory will report the reference ranges to you along with your results. You’ll get these whether your doctor orders a thyroid test and you go to a lab to have it performed, or whether you order a home test kit and send in a blood sample (obtained by a fingerprick) for your thyroid testing. Simply compare your results to the reported reference ranges in order to determine whether your levels are low or high. If your numbers are within the ranges, then you’re considered to be in the normal range for thyroid levels. If they’re outside of these, then you’ll need to talk with a medical professional to discuss treatment options.
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What do TSH levels mean?
TSH is the most commonly used screening test for thyroid problems. In general, when you go for your annual physical and receive screening bloodwork, TSH will usually be the test that’s done. In some cases, a “reflex free T4” test is used. For this type of test, TSH is tested first. If it’s normal, then the test is over, and no further testing is considered necessary. If it’s abnormal, then free T4 will also be tested.
If the pituitary is healthy, then TSH will generally react to thyroid problems by moving in the opposite direction. When thyroid hormone levels are low, the pituitary makes more TSH. When thyroid hormone levels are high, then TSH levels will drop. The pituitary is extremely sensitive to changes in thyroid hormone levels, and TSH will react to even small changes in the production of thyroid hormones.
There are also some cases where abnormal thyroid hormone levels actually result from a problem with the pituitary, and the thyroid gland itself is actually normal. In this case, the TSH and free T4 levels would go together. For example, if TSH and free T4 are both low, this indicates hypothyroidism that’s caused by an issue with the pituitary gland, which is not secreting the proper levels of hormones to stimulate the thyroid gland.
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Normal thyroid levels in women
Average thyroid levels are somewhat different in women and men. Women with a normal level of thyroid function tend to have a slightly higher TSH level, and slightly lower free T4, than men. This is believed to be related to their estrogen levels; estrogen raises the level of thyroid binding globulin (TBG) in the blood, which results in more T4 being bound to this carrier protein and less T4 being “free” and available for the body to use. Because free T4 is lower, TSH rises.
In pregnancy, TSH levels are generally significantly lower; the specific reference range varies by the trimester of pregnancy, so your doctor will talk with you about any findings that are abnormal for where you’re at in the pregnancy.
Again, the specific reference range can differ for different laboratories, depending on their testing techniques, so check the ranges for your particular lab.
Normal thyroid levels in men
For men, average TSH levels are slightly lower than for women, and free T4 levels tend to be slightly higher. Age also has an impact. In general, in both men and women, the production of thyroid hormones goes down as people age, and TSH levels generally rise.
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Sources
Hypothyroidism (underactive thyroid) Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/hypothyroidism/symptoms-causes/syc-20350284. Accessed 25 May 2022.
Hyperthyroidism (overactive thyroid). Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/hyperthyroidism/symptoms-causes/syc-20373659. Accessed 25 May 2022.
Meng Z, Liu M, et al. Gender and Age Impact on the Association Between Thyroid-Stimulating Hormone and Serum Lipids. Medicine (Baltimore). 2015 Dec;94(49):e2186. doi: 10.1097/MD.0000000000002186.
Park SY, Kim HI, et al. Age- and gender-specific reference intervals of TSH and free T4 in an iodine-replete area: Data from Korean National Health and Nutrition Examination Survey IV (2013–2015). PLoS One. 2018; 13(2): e0190738.doi: 10.1371/journal.pone.0190738.
Song Q, Chen X, et al. Age and Gender Specific Thyroid Hormones and Their Relationships with Body Mass Index in a Large Chinese Population. Int J Endocrinol Metab. 2019 Jan;17(1):e66450. doi: 10.5812/ijem.66450.
