High and Low TSH Levels: What They Mean

TSH Is the Key Test for Diagnosing and Monitoring Thyroid Function

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Interpreting TSH levels is not necessarily intuitive, so you are not alone if you're wondering what your thyroid levels mean, and specifically what high and low TSH levels mean for treatment.

For instance, you may question why your doctor wants to lower instead of raising your thyroid medication when your TSH results are low, or, conversely, why your doctor increases your thyroid medication when your TSH is high.

Rest assured, that while it may seem backwards, it all makes sense when you look at the biology of thyroid hormone production.

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. 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 reveal about your thyroid's function.

    Since TSH raises thyroid hormone levels and keeps the system in normal balance:

    • A high TSH suggests your thyroid 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 your thyroid 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, without also evaluating the circulating levels of actual thyroid hormones, like free thyroxine (T4), may not be able to detect more subtle thyroid problems.

    For instance, free T4 in addition to TSH is also usually tested if a doctor suspects thyroid dysfunction arising from disease of the pituitary gland or hypothalamus. Likewise, if the TSH is normal, but a person still has symptoms of being hyperthyroid or hypothyroid, free T4 may be checked.

    TSH is also not necessarily sufficient to monitor hypothyroidism during pregnancy, which is why free T4 and/or total T4 are also checked.

    Depending on the clinical situation, other thyroid tests that may be evaluated include triiodothyronine (T3), reverse T3, and antibody 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 0.4 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 to 0.4 to 2.5mU/L.

    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.

    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. This is because excessive suppression of TSH (meaning high thyroid hormone production) can increase a person's risk of atrial fibrillation (a heart arrhythmia) or osteoporosis.

    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).

    A Word From Verywell

    In summary, the TSH test is the blood test doctors primarily utilize to both diagnose hypothyroidism and hyperthyroidism and monitor thyroid hormone replacement therapy (if needed).

    While medical situations, like pregnancy or being hospitalized, may require that the T4 and T3 be measured, grasping the basic concept of what a high or low TSH levels ​mean for your thyroid function is really the meat of what you need to know.

    That said, if you have any questions about your thyroid-related blood work, be sure to ask your doctor.

    View Article 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.
    • Ross DS. (2017). Laboratory assessment of thyroid function. In: UpToDate, Cooper DS (Ed), UpToDate, Waltham, MA.