High and Low TSH Levels: What They Mean

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Interpreting TSH levels and choosing appropriate treatment tends to be confusing. You're likely wondering what your thyroid levels mean, and specifically what high and low TSH levels mean for treatment. Why, when your TSH results show low TSH, does your doctor want to lower instead of raise your medicine? And if a high TSH means you have too much thyroid, why is the doctor increasing your thyroid hormone medicine?

Thyroid Basics

Your thyroid gland produces thyroid hormone. When it functions properly, your thyroid is part of a feedback loop with your pituitary gland that involves several key steps.

  1. First, your pituitary senses the level of thyroid hormone that is released into the bloodstream.
  2. Your pituitary releases a special messenger hormone: thyroid stimulating hormone (TSH). The role of TSH is to stimulate the thyroid to release more thyroid hormone.
  3. When your thyroid, for whatever reason—illness, stress, surgery, or obstruction, for example—doesn't or can't produce enough thyroid hormone, your pituitary detects the reduced levels of thyroid hormone and moves into action by making more TSH, which then triggers your thyroid to make more thyroid hormone. This is the pituitary's effort to raise the levels of thyroid hormone and return the system to normal.
  4. If your thyroid is overactive and producing too much thyroid hormone—due to disease, or taking too high a dose of thyroid hormone replacement drugs—your pituitary senses that there is too much thyroid hormone circulating and slows or shuts down TSH production, so that the thyroid will slow down its production of the hormone. This drop in TSH is an attempt to return circulating thyroid hormone levels to normal.

    Interpreting TSH Levels

    Once you understand these thyroid basics, it's easier to understand what a low TSH and a high TSH reveals about your thyroid's function. Since TSH raises thyroid hormone levels and keeps the system in normal balance:

    • A high TSH suggests a thyroid that is underactive (hypothyroid) and not doing its job of producing enough thyroid hormone. The excess TSH is working to stimulate your thyroid to produce more thyroid hormone.
    • A low TSH suggests a thyroid that is overactive (hyperthyroid) and producing excess thyroid hormone. The pituitary gland is suppressing TSH so that the thyroid doesn't produce even more.

    Is TSH Reliable?

    During diagnosis, most doctors use the TSH test to evaluate your thyroid function and determine the optimal course of treatment. Note, however, that some practitioners feel that relying solely on TSH—a pituitary hormone—without also evaluating the circulating levels of actual thyroid hormones T4 and T3—may not be able to detect more subtle thyroid problems, or conditions that are resulting from the improper conversion of thyroid hormones.

    TSH is also not necessarily sufficient to monitor hypothyroidism during pregnancy. For these reasons, some practitioners also include other valuable blood tests, including T4, T3, Free T4, Free T3, reverse T3, and antibodies tests.

    TSH Reference Ranges

    A major hitch in the connection of TSH to hypothyroidism and hyperthyroidism is an ongoing disagreement in the medical world over the reference range for the TSH test. Levels below 0.5 are considered possible evidence of hyperthyroidism, and levels above 5.0 are typically considered possible evidence of hypothyroidism, but some experts feel this range is too broad and that it should be narrowed significantly, to 0.3 to 3.0.

    This is an issue that is still under debate.

    Determining Treatments Based on TSH

    When you are being treated for hypothyroidism with thyroid hormone replacement drugs, doctors will typically attempt to medicate you into the "normal" reference range of a TSH from 0.3/0.5 on the low-end, to 3.0/5.0 on the high-end. Patients who have had thyroid cancer, however, are often given suppressive doses that maintain TSH near to 0 in order to prevent cancer recurrence.

    So, when you've gone for a checkup and your TSH comes in below normal (which means that TSH is being suppressed because thyroid hormone levels are already high), your doctor may want to reduce your dosage of thyroid hormone because you are already hyperthyroid.

    And if your TSH test comes in above normal, some doctors will want to increase your dosage of thyroid hormone, because levels above normal are considered potentially hypothyroid (underactive).

    Sources:

    Bahn, R., Burch, H, Cooper, D, et al. Hyperthyroidism and Other Causes of Thyrotoxicosis: Management Guidelines of the American Thyroid Association and American Association of Clinical Endocrinologists. Endocrine Practice. Vol 17 No. 3 May/June 2011.

    Braverman, L, Cooper D. Werner & Ingbar's The Thyroid, 10th Edition. WLL/Wolters Kluwer; 2012.

    Garber, J, Cobin, R, Gharib, H, et. al. "Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association." Endocrine Practice. Vol 18 No. 6 November/December 2012.

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