What is TSH (Thyroid Stimulating Hormone)?
Thyroid Stimulating Hormone (TSH) is produced by your pituitary gland and is the throttle for your thyroid. Your thyroid regulates every single cell in your body with thyroid hormones: thyroxine 4 (T4) and triiodothyronine (T3).
When your body needs to produce more thyroid hormone, your pituitary steps on the gas pedal by producing more TSH, which in turn tells your thyroid to produce more hormone. When your body needs less thyroid hormone, your pituitary produces less TSH, which signals the thyroid to slow down hormone production.
It’s important to know what your TSH level is because it indicates what your thyroid is doing (or not doing). For example:
- If TSH levels are HIGH, it indicates that thyroid production may be LOW
- If TSH levels are LOW, it indicates that thyroid production may be HIGH
- Abnormal TSH levels are an indicator of thyroid disease
We care about TSH because of its close relationship with our thyroids.
What a TSH Test Is Used For
- What is a TSH Test?
- Understand your TSH Test Results
- Normal TSH Levels and Why Normal Isn’t Always Good
TSH tests have been the most common test screening for thyroid function for over a decade . is used to test the amount of Thyroid Stimulating Hormone that our pituitary gland is producing. Indirectly, we use a TSH tests as an indicator of:
- What your thyroid is doing
- How much thyroid hormone is floating around in your body (TSH decreases when T4 rises, and vice versa)
As a general rule:
- High TSH tells us that your pituitary is pressing on the gas pedal, but your body isn’t producing enough T3 and T4
- Low TSH tells us that your thyroid is producing too much T3 and T4
High TSH is a symptom of hypothyroidism. Low TSH is a symptom of hyperthyroidism. Usually, these tests are done in response to symptoms that you are experiencing, though functional medicine practitioners often highly recommend TSH testing at the onset of pregnancy because thyroid problems.
How to Test TSH
Testing your TSH levels is as simple as getting a blood draw. Also, contrary to what many sites report, even the largest medical sites such as webmd, research shows us that time of day and fasting are relevant to your TSH test, and the best times to establish your upper TSH level may be in the morning on an empty stomach.
TIME OF DAY: TSH tests DO fluctuate with the time of day TSH levels peak in the middle of the night, and so getting tested in the morning will be helpful to see what your TSH upper limit is .
FASTING: You will also commonly see online that you don’t need to fast prior to a TSH test. However, research shows, such as this 2014 study, that TSH levels decline significantly after meals in comparison with fasting levels .
OTHER FACTORS: It’s important to note that if you have received radioactive materials or had X-rays that used iodine dye in the 4-6 weeks preceding your test, your results may not be correct.
THYROID MEDICATION: There are mixed opinions about whether or not you should take thyroid medication before your TSH test. Consult your medical practitioner to see what they recommend and why they recommend it.
TSH Test Results: Normal TSH Levels? (There’s a Better Question)
The first questions you care about when you receive your TSH test results is, “Is this normal?” Unfortunately, there’s no consensus on exactly what normal TSH levels are in terms of the upper limit. However:
- .45 – 5.5 mIU/L | Historically this has been the accepted range of TSH [4, 5, 6]
- .45 – 4.5 mIU/L | In 2008 the Endocrine Society and American Medical Association adopted this range [4, 5, 6]
- < 3.0 mIU/L | The American Association of Clinical Endocrinologist now recommends this as an upper limit 
- < 2.5 mIU/L | National Academy of Clinical Biochemistry suggests this as an upper limit for TSH levels , as 95% of normal individuals have TSH levels below 2.5 
However, a much better question to ask is what is “Are my TSH levels functional?” “Functional” TSH level addresses whether or not your thyroid is working optimally. It’s sort of like the difference between getting a C- and an A-. C- isn’t failing grade, but do you want it?
Many functional medicine providers suggest that an optimal range for functional TSH levels is 1.0 – ~2.0 mIU/L. This is because:
- Studies have found that TSH above 2.0 may be associated with cardiovascular risk factors , as well as other serious health risks such as dysglycemia (above 2.1 mIU/L)  and non-alcoholic fatty liver disease (NAFLD) ,
- TSH between 1.0 and 2.0 has been associated with the lowest incidence of abnormal thyroid function 
Remember, TSH levels fluctuate throughout the day and over time depending any number of factors (stress level, infection, etc.). If your TSH levels are high, you may consider retesting later to hone in on an accurate diagnoses.
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 Rakesh Nair, Shriraam Mahadevan, R. S. Muralidharan, and S. Madhavan. Does fasting or postprandial state affect thyroid function testing? Indian J Endocrinol Metab. 2014 Sep-Oct; 18(5): 705–707.
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