What is a Normal Thyroid Stimulating Hormone (TSH) level?

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I recently saw a new patient getting ready for IVF. Her TSH was 2.90. I advised further thyroid testing and started her on Thyroid supplementation. Why?

The thyroid is a butterfly-shaped gland that is located in the front of the neck.

The thyroid’s main function is to regulate our metabolism (the body’s ability to convert food into fuel, as well as how efficiently you use the fuel).

What is thyroid disease?

Thyroid disease is a common condition. It has been estimated that as many as 2 percent to 7 percent of Americans have some form of thyroid disease. Women are 5-8 times more likely to experience thyroid disease than men before age 50.

Doctor's chart and stethoscope | RSC Bay Area | CA

The most common test done to assess thyroid function is TSH (Thyroid Stimulating Hormone). TSH is produced by our pituitary gland. TSH stimulates the thyroid to produce the hormones T4 (thyroxine) and T3 (triiodothyronine).

If the thyroid gland in underactive, it is called hypothyroidism. When the thyroid gland is overactive, this is referred to as hyperthyroidism.

A number of conditions may affect the thyroid gland’s function including:

  • Autoimmune conditions (conditions where the body attacks itself) including thyroiditis
  • Pregnancy
  • Steroids
  • Medications (Lithium, Hormones including Estrogen, Potassium Iodide)

Normal thyroid function is needed to ensure the best chance for a successful conception and pregnancy. During pregnancy, maternal thyroid hormone production increases by 20 percent to 40 percent. A number of medical studies have concluded that normal thyroid function before and during pregnancy can reduce the risks of miscarriage, preterm (early) labor and avoid potential adverse effects on normal mental development on the fetus. Thyroid hormone is extremely important in normal fetal nervous system development. The developing fetus is not able to regulate and produce adequate thyroid hormone until 20 weeks of pregnancy.

Treating thyroid disease

In most clinical laboratories in the U.S., the “normal” reference range for the TSH is 0.4 to 4.50 mIU/liter. If the TSH is above the “normal” reference range, this may indicate hypothyroidism. In hyperthyroidism, the TSH is below the “normal” range or not detected at all. If the TSH level is abnormal, further testing is often done measuring the T4 and T3 levels along with thyroid antibody testing. In January 2003 the American Association of Clinical Endocrinologists advised that doctors “consider treatment for patients who test outside the boundaries of a narrower margin based on a target TSH level of 0.3 to 3.0. While controversial, many experts now consider a TSH above 2.5 as abnormal.

Treatment guidelines for women with thyroid disease typically advise keeping the TSH level between 0.1 and 2.5 in the first trimester. A recently published study in the Journal of Clinical Endocrinology and Metabolism concluded that the upper limit of TSH in the first 12 weeks of pregnancy should be 2.5.

Given the opportunity to reduce obstetrical risks and avoid long-term neurological issues in the children, the benefits of bringing the TSH level into the target range of 0.1 to 2.5 appear to outweigh any potential risks associated with thyroid medications.

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