TSH (Thyroid-Stimulating Hormone): Normal Range, What High & Low Levels Mean

TSH stands for Thyroid-Stimulating Hormone, also called thyrotropin. Your pituitary gland — a small gland at the base of your brain — produces it and releases it into your blood to signal your thyroid to make thyroid hormones. The TSH blood test measures how much of this signaling hormone is present in your bloodstream at the time of the draw. Because TSH rises and falls in response to what your thyroid is doing, it acts as a highly sensitive early-warning indicator for thyroid problems, even before obvious symptoms appear. Talk with your healthcare provider about what your specific TSH result means for you.

Normal TSH ranges depend on your age and whether you are pregnant. For adults aged 18 to 60, the typical reference range is 0.4 to 4.0 mIU/L. For adults over 60, the accepted range extends slightly to 0.4 to 4.5 mIU/L. During the first trimester of pregnancy, a tighter range of 0.1 to 2.5 mIU/L applies, reflecting how important stable thyroid function is for a developing baby. Keep in mind that laboratories can have slightly different reference intervals, so the range printed on your own lab report is the most relevant benchmark for your result. Your healthcare provider is the right person to interpret where your number falls for your individual situation.

A TSH result above the normal range usually means your thyroid is underactive — a condition called hypothyroidism. When the thyroid isn't producing enough hormones on its own, the pituitary gland compensates by releasing more TSH, effectively turning up the signal. The most common cause of hypothyroidism in developed countries is Hashimoto's thyroiditis, an autoimmune condition. Other causes include thyroid surgery, radioactive iodine treatment, certain medications such as lithium or amiodarone, iodine deficiency, or, rarely, a TSH-producing tumor in the pituitary gland. A high TSH alone is not a diagnosis — your healthcare provider will review your full picture before recommending any next steps.

A TSH result below the normal range often suggests your thyroid is overactive — a condition called hyperthyroidism. When the thyroid produces too many hormones, the pituitary gland reduces its TSH output in response. The most common cause is Graves' disease, an autoimmune condition. Other causes include a toxic multinodular goiter, taking too much thyroid hormone medication, or damage to the pituitary gland itself. During early pregnancy, hCG — the hormone detected by pregnancy tests — has mild TSH-like activity that can naturally lower TSH in the first trimester. Certain medications, including corticosteroids and dopamine agonists, can also suppress TSH. Discuss your specific low result with your healthcare provider to understand what it means for you.

Symptoms differ depending on whether your TSH is high or low. A high TSH — pointing toward an underactive thyroid — may accompany fatigue, unexplained weight gain, feeling cold all the time, constipation, dry skin or hair loss, a slow heart rate, low mood, or a puffy face and swollen legs. A low TSH — suggesting an overactive thyroid — may come with unintentional weight loss, a rapid or irregular heartbeat, feeling overheated and sweating excessively, nervousness or tremor in the hands, more frequent bowel movements, or an enlarged thyroid gland. Some people have abnormal TSH levels with no noticeable symptoms at all. Your healthcare provider can help you connect your symptoms to your specific result.

How often you should retest TSH depends on your personal health history, whether you are on any medications that affect the thyroid, and what your previous results have shown. Someone newly diagnosed with a thyroid condition may need more frequent monitoring while a treatment plan is established, while someone with consistently stable results may only need periodic checks. Pregnancy also changes how frequently testing is recommended. Factors like new symptoms, a change in medication, or a significant life event such as surgery can all be reasons your doctor might want to recheck your TSH sooner than scheduled. Your healthcare provider is best placed to advise on the right retesting schedule for your circumstances.