Normal thyroid-stimulating hormone (TSH) levels generally fall between 0.5 and 5.0 milliunits per liter (mU/L).
However, factors such as pregnancy can affect what a “normal” TSH level is.
Illustration by Emily Roberts for Verywell Health
How TSH Levels Change
TSH results may seem counterintuitive.
Illustration by Emily Roberts for Verywell Health
A high TSH level suggests an underactivethyroid gland, while a low TSH levels indicate an overactive thyroid.
TSH influences the production of thyroid hormones, but they are different things.
The thyroid hormones include:
Your body uses thyroid hormones to regulate all kinds of processes.
When thyroid hormones are low, the pituitary makes more TSH, which signals the thyroid to increase production.
When levels are high, the pituitary releases less TSH, which signals the thyroid to slow down.
This is why high TSH indicates low thyroid function and vice versa.
TSH Levels by Sex
TSH levels vary by sex.
Low estrogen impairs the thyroid gland’s ability to make thyroid hormones.
So, TSH rises to get the thyroid to produce more.
High estrogen leads to increased thyroid hormone production and lower TSH.
Inmenopause, estrogen levels are lower, which sometimes triggers thyroid disease.
TSH Levels During Pregnancy
During pregnancy, TSH levels vary by trimester.
Many healthcare providers sidestep this controversy by looking at each person individually.
The pituitary gland senses these low levels and increases the production of TSH.
A low TSH levelbelow 0.5 mU/Lindicates an overactive thyroid, also known as hyperthyroidism.
This means your body is producing an excess amount of thyroid hormone.
This deficiency leads to several other symptoms as well.
Some of the possible reasons for variations in TSH results can include the following.
There also are variations in the reference range used by providers, as well as test procedures.
Summary
An optimal TSH level for all people cannot reasonably be established.
Your healthcare provider will consider these factors and any symptoms you’re experiencing to determine your optimal range.
The TSH test is the gold standard in diagnosing and monitoring thyroid disease.
Your healthcare provider may order additional tests to help determine how your thyroid gland is performing.
American Thyroid Association.Thyroid Function Tests.
Oklahoma Heart Hospital.Thyroid Health and Heart Rhythm Disorders: Understanding the Connection.
2018 Jul;39(5):830-850. doi:10.1210/er.2018-00119
National Institute of Diabetes and Digestive and Kidney Diseases.Thyroid tests.
2018 Jun;10(6):e2845.
2019 Dec;12(1):369-374. doi:10.2147/RMHP.S234552
