The table below provides estimates of TSH levels that are normal, low (indicating hyperthyroidism) and high (indicating hypothyroidism):

Most labs use these reference values.

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TSH stands for thyroid stimulating hormone, though it is sometimes called thyrotropin or thyrotropic hormone. TSH stimulates the thyroid gland to produce thyroid hormone, which is is critical for the proper function of virtually every cell in the body. TSH is released by the pituitary gland after the gland has been stimulated by thyroid releasing hormone (TRH), which is secreted by the hypothalamus. Thyroid hormone provides negative feedback on the hypothalamus and/or the pituitary to reduce thyroid hormone production and release. TSH measurements are important for diagnosing hypothyroidism and hyperthyroidism. TSH may be measured in conjunction with total thyroxine (T4), total triiodothyronine (T3), free T4, free T3, and reverse T3 concentrations in the serum.

Normal Ranges for TSH:

Children:

- 1-2 Days --- 3.20-34.60 mIU/L

- 3-4 Days --- 0.70-15.40 mIU/L

- 5 Days-4 Weeks --- 1.70-9.10 mIU/L

- 1-11 Months --- 0.80-8.20 mIU/L

- 1-19 Years --- 0.50-4.30 mIU/L

Adult (non-pregnant):

0.40-4.50 mIU/L

Pregnancy  

- First Trimester --- 0.26-2.66 mIU/L

- Second Trimester --- 0.55-2.73 mIU/L

- Third Trimester --- 0.43-2.91 mIU/L

Factors That May Affect Your TSH Results:

There are a number of variations and factors that can affect TSH levels. It's important to be aware of these, as treatment that is dictated solely by lab values (as opposed to also considering an individual's symptoms) can result in an ineffective plan.

- Laboratory Error: If a TSH level is surprising, sometimes simply repeating the test is the best course. Errors can occur during the blood draw, in transcribing the results, or due to mix-ups in the lab. Statistically, there is always a risk of lab error, and results should always be interpreted along with clinical symptoms and findings.

- Antibodies: Antibodies are thought to interfere with accurate thyroid testing in roughly 1 percent of people. In a 2018 review, it was estimated that in people who have these antibodies, the interference with TSH testing caused either misdiagnosis or inappropriate treatment in more than 50 percent of cases:

Heterophile antibodies: Heterophile antibodies are antibodies that may occur when a person is exposed to animal-derived pharmaceuticals and antibody therapies. Their presence is more common in people who have had certain vaccinations, blood transfusions, or have been exposed to some animals (not household pets). The estimated incidence of these antibodies varies widely, but when present, they can interfere with TSH levels. There is no easy way to know if you have these antibodies, but a discrepancy between TSH levels and free T4 (the hallmark of heterophile antibodies), or between TSH levels and how you feel, should raise the question.

Thyroid antibodies: Thyroid autoantibodies, present in some people with or without a thyroid condition, may also affect TSH levels. Again, a discrepancy between lab values and how you feel should raise the question of whether or not the test is accurate.

Other antibodies: Other antibodies important in TSH testing interference include anti-ruthenium antibodies and anti-streptavidin antibodies.

- Other Factors: A number of other factors can affect TSH test results either through having an effect on actual levels of thyroid hormones or interacting with testing measures. 

Some of these include:

- The time of day that the test is done: TSH levels are higher if you're tested after fasting (for example, in the morning after not having eaten since the night before) as compared to after eating later in the day.

- Illness

- Pregnancy

- Some medications that are used for heart disease and in cancer treatment

- Foods or supplements rich in/derived from iodine or kelp

- Biotin supplements

- Non-steroidal anti-inflammatory medications such as Advil (ibuprofen)

- Changes in sleep habits

In order to get the most accurate results, it's important to be consistent. For example, always having your test done at the same time of day.

Sources:

http://www.uptodate.com/contents/thyroid-hormone-synthesis-and-physiology

http://www.ncbi.nlm.nih.gov/pubmed/2194786

http://www.uptodate.com/contents/laboratory-assessment-of-thyroid-function

http://www.questdiagnostics.com/testcenter/TestDetail.action?ntc=899

http://www.uptodate.com/contents/disorders-that-cause-hypothyroidism

http://www.uptodate.com/contents/disorders-that-cause-hyperthyroidism

http://www.ncbi.nlm.nih.gov/pubmed/21048053

http://www.nejm.org/doi/full/10.1056/NEJMcp0801880

http://www.uptodate.com/contents/disorders-that-cause-hyperthyroidism