TSH (Thyroid-Stimulating Hormone) levels, explained
TSH is the pituitary hormone that tells your thyroid how hard to work — it is the primary screening test for thyroid function.
What’s a normal TSH level?
Typical adult reference range, shown for orientation. Your report’s range may differ by lab, age, and sex — the analyzer uses your report’s own ranges when available.
What high and low TSH mean
A value outside the reference range is a flag, not a diagnosis. Here’s what each direction usually points to — and the most common causes.
High TSH typically indicates hypothyroidism (an underactive thyroid) — the pituitary is shouting at a thyroid that is not keeping up.
- Hashimoto’s (autoimmune) thyroiditis — the most common cause
- Iodine deficiency or certain medications
- Under-replacement in people already on thyroid medication
- Recovery phase after thyroid inflammation or surgery
Low TSH usually indicates hyperthyroidism (an overactive thyroid), where excess thyroid hormone suppresses the pituitary signal.
- Graves’ disease (autoimmune hyperthyroidism)
- Toxic nodules or multinodular goiter
- Over-replacement with thyroid medication
- Early thyroiditis (a temporary hormone surge)
When a TSH result needs attention
A markedly high TSH with symptoms (fatigue, weight gain, cold intolerance) or a very suppressed TSH with palpitations, tremor, or weight loss should be evaluated promptly with free T4/T3 testing.
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TSH (Thyroid-Stimulating Hormone) — frequently asked questions
- Does a high TSH mean hypothyroidism?
- Usually yes. A high TSH means the pituitary is working overtime to stimulate a thyroid that is underproducing — the hallmark of hypothyroidism. It is confirmed by checking free T4 and, where relevant, thyroid antibodies. Mildly elevated TSH with a normal free T4 is called subclinical hypothyroidism and is often monitored rather than immediately treated.
- What does a low TSH indicate?
- A low or suppressed TSH usually points to hyperthyroidism — an overactive thyroid producing too much hormone, which switches off the pituitary signal. Causes include Graves’ disease, thyroid nodules, and over-replacement with thyroid medication. Symptoms can include palpitations, tremor, weight loss, anxiety, and heat intolerance.
- What is the normal range for TSH?
- Most labs use roughly 0.4–4.0 mIU/L for adults. Optimal is often considered closer to 1–2.5 mIU/L. Pregnancy, age, and the specific assay shift the target, so interpret TSH against the reference range printed on your own report.
Related lab markers
- Hemoglobin →Hemoglobin is the protein in red blood cells that carries oxygen — low hemoglobin defines anemia.
- Vitamin D (25-Hydroxyvitamin D) →The 25-hydroxyvitamin D test is the standard measure of your vitamin D status, reflecting both sun exposure and dietary intake.
- Ferritin →Ferritin is the protein that stores iron in your body — it is the single best blood marker of how much iron you have in reserve.
- All lab markers →Browse every biomarker guide in one place.
This page provides educational health information and does not replace professional medical advice, diagnosis, or treatment. Interpret any lab value with your clinician, who has your full medical context. For emergencies, contact emergency services.