What Is TSH in a Blood Test?

TSH stands for Thyroid-Stimulating Hormone. It is produced by the pituitary gland in your brain and signals your thyroid gland to release T3 and T4 hormones that regulate your metabolism, heart rate, body temperature, and energy levels.

Normal Range

• Adult normal range: 0.4–4.0 mIU/L (varies by lab; some guidelines now consider 0.5–2.5 mIU/L as optimal)
• Pregnancy ranges differ; trimester-specific reference values are used.

What High TSH Means

A high TSH indicates the pituitary is working harder than normal to stimulate the thyroid — usually because the thyroid is underactive or not responding adequately. This is called hypothyroidism. Common causes include Hashimoto's thyroiditis (autoimmune), iodine deficiency, certain medications (lithium, amiodarone), and radiation therapy to the neck. Symptoms of high TSH / hypothyroidism include fatigue, weight gain, cold intolerance, dry skin, constipation, and depression.

What Low TSH Means

A low TSH indicates the pituitary is reducing its signal because there is already too much thyroid hormone circulating — hyperthyroidism. Common causes include Graves' disease (autoimmune), toxic multinodular goiter, thyroiditis, or excessive thyroid hormone medication. Symptoms include palpitations, weight loss despite increased appetite, heat intolerance, tremors, anxiety, and insomnia.