The normal range of TSH levels in non-pregnant adult women is 0.5 to 5.0 mIU/L.
In women, during the menstruation cycle, pregnancy, or after menopause, TSH levels may fall slightly outside the normal range, because of fluctuating levels of estrogen.
- The normal range of TSH levels in non-pregnant adult women is 0.5 to 5.0 mIU/L.
- During the first trimester of pregnancy, total T3 and T4 levels go up and TSH levels fall. Free T4 and T3 also are high-normal in the first trimester and return to normal in the second trimester.
- Women, however, are five to eight times likelier than men to develop thyroid conditions, which may possibly be linked to a higher incidence of autoimmune disease in women. Thyroid disease can cause problems with the menstrual cycle, getting pregnant, and in carrying pregnancy safely to full term.
- Postpartum thyroiditis (thyroid inflammation), an autoimmune thyroid disease that occurs during the first year after delivery, is reported to affect up to 10% of women. Postpartum thyroiditis often resolves on its own. Autoimmune thyroid diseases tend to improve during pregnancy, possibly due to the altered state of immunity during pregnancy.
What do abnormal TSH levels mean?
Abnormal TSH levels can indicate thyroid dysfunction or pituitary gland disease. Abnormal TSH and thyroid hormone levels commonly indicate the following conditions:
Primary hypothyroidism
High TSH, low thyroid hormones
Subclinical hypothyroidism
High TSH, normal thyroid hormones
Primary hyperthyroidism
Low TSH, high thyroid hormones
Subclinical hyperthyroidism
Low TSH, normal thyroid hormones
Thyroiditis
Low TSH, high thyroid hormone followed by
high TSH, low thyroid hormone
Pituitary disease
Low TSH, low thyroid hormones
Early or mild hyperthyroidism
Low TSH, normal thyroid hormone level
What are thyroid hormones?
Thyroid hormones are secreted by the thyroid gland using iodine, a mineral that is present in many foods and is abundant in seafood. The thyroid gland is a butterfly-shaped endocrine gland located at the base of the front of the neck. Endocrine glands are a network of organs that produce and release various hormones necessary for the normal functioning of the body.
Thyroid hormones regulate the metabolic rate of the body and thus play a major role in the proper functioning of every organ in the body. Thyroid hormones are essential for normal brain and physical development in infants. Thyroid hormones regulate body temperature and help maintain the normal function of all organs including the brain, heart, and muscles.
Thyroid hormones are found in two forms in the body:
- Thyroxine (T4): Thyroxine constitutes approximately 95% of thyroid hormone circulating in the blood.
- Triiodothyronine (T3): T3 accounts for the balance of 5% of thyroid hormone and is formed by the removal of one atom of iodine from T4, which is mostly done in the liver or kidney.
Only a minuscule percentage of thyroid hormones are in a free form that can enter cells and activate metabolism. More than 99.95% of T4 and 99.5% of T3 are bound to proteins in the blood. The binding proteins act as a storage system for the thyroid hormones, regulating the availability of free T4 and T3 for metabolic function.
What is thyroid stimulating hormone (TSH)?
Thyroid-stimulating hormone (TSH) is a hormone that stimulates the thyroid gland to produce thyroid hormones. The pituitary gland secretes more TSH when blood levels of T4 and T3 fall below normal, and reduces the TSH secretion when T4 and T3 levels rise. The hypothalamus region of the brain regulates pituitary gland activity.
What is a normal TSH level in adults?
Normal TSH levels may vary in adults depending on age, sex, certain medical conditions, or medications. Stress and diet also may play a role in fluctuating TSH levels.
In some situations, the endocrinologist may optimally maintain a different range of TSH levels, for example, in a person of advanced age, or a patient with a history of thyroid cancer or pituitary gland disorder.
Following are the normal ranges for the TSH and thyroid hormone levels in an adult, which are measured in blood tests to determine if the thyroid function is normal:
- TSH level: TSH level in an adult is normally within the range of 0.5 to 5.0 mIU/L (milli-international units per liter). A TSH level higher than 5.0 usually indicates an underactive thyroid (hypothyroidism) and a TSH level lower than 0.4 indicates the presence of excessive thyroid hormone and overactive thyroid (hyperthyroidism). The normal range may vary slightly in different laboratories.
- Total T4 level: This test measures both protein-bound and free T4. A normal total T4 level in an adult is in the range of 5.0 to 12.0 mcg/dL (microgram/deciliter). High T4 indicates hyperthyroidism and low T4 indicates hypothyroidism.
- Free T4 (FT4) level: FT4 test may be a more accurate way to determine thyroid function because, unlike total T4, free T4 level is not affected by the binding proteins. Normal FT4 level falls in the range of 0.7 to 1.9 ng/dL (nanogram/deciliter).
- Total T3 level: Total T3 tests are done to confirm hyperthyroidism if total T4 is normal, but hyperthyroidism symptoms are present. Total T3 in an adult is within the range of 80 to 220 ng/dL. Free T3 may be measured sometimes, but the results are often not as reliable.
If the above tests produce inconclusive results, the individual may undergo additional tests to determine thyroid function, which may include:
- T3 resin uptake (T3RU) test: A measurement of protein binding capacity of the thyroid hormones.
- Free T4 index (FT4I) test: The proportion of free T4 in comparison with protein-bound T4.
Other tests to detect the presence of various antibodies which are helpful in diagnosing autoimmune thyroid disorders include the following:
- Thyroid-stimulating immunoglobulin test
- Antimicrosomal antibody test
- Anti-thyroid peroxidase antibody test
- Antithyroglobulin antibody test
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What are non-thyroid causes for abnormal TSH levels?
Certain medications, supplements, non-thyroid illnesses, and other factors can produce abnormal thyroid test results which do not match symptoms. It may be a good idea to repeat the thyroid tests and/or investigate what else might cause the abnormal TSH levels.
Some possible non-thyroid causes for transient changes in TSH levels include the following:
- Illnesses such as infection, cancer, kidney disease, or heart failure.
- Medications used for heart disease or cancer.
- Fasting or starvation can cause low TSH.
- Biotin, a commonly used supplement for hair and nail growth can interfere with thyroid test results.
- People who are exposed to mice, for instance, lab researchers and veterinarians, may develop antibodies to mouse proteins and may produce unpredictable results in thyroid tests.
What TSH level requires treatment?
The American Thyroid Association recommends that patients older than 65, should be treated for hyperthyroidism if TSH levels are below 0.1 mIU/L. Patients younger than 65 are treated if they are post-menopausal women, have coexisting conditions (comorbidities) such as heart disease or osteoporosis, or symptoms of hyperthyroidism.
Subclinical hyperthyroidism with TSH levels between 0.1 to 0.4 mIU/L with normal T3 and T4 levels may be just monitored if the patient is below 65 and has no symptoms. Patients above 65 or those below 65 with comorbidities or symptoms of hyperthyroidism may be treated depending on their condition.
Doctors usually start treatment for hypothyroidism if the TSH level remains persistently elevated above 10.0 mIU/L even if T4 is normal and the patient has no symptoms. Thyroid levels are usually periodically monitored and maintained optimally with appropriate dosage changes.
Subclinical hypothyroidism, which means TSH levels of 5.0 to 10.0 mIU/L and normal T4, may require treatment if the individual has a positive thyroid antibodies test, experiences symptoms, or is at high risk for developing hypothyroidism.
Growing evidence from research suggests that mild elevations in TSH levels can be monitored with periodic tests and may not require treatment. In some situations, as in pregnancy, an elevated TSH level may resolve on its own. TSH levels have also been found to gradually increase with age, and mildly elevated TSH levels in people above 80 do not appear to increase mortality.
If you have questions about TSH levels, ask your doctor to explain what your TSH level means.