What Is a TSH Blood Test?

A TSH blood test measures Thyroid-Stimulating Hormone — produced by the pituitary gland to signal the thyroid. It is the most sensitive test for detecting thyroid dysfunction.

Normal Range

• 0.4–4.0 mIU/L
• Some endocrinologists prefer 0.5–2.5 mIU/L as optimal

Why TSH Is the Best Screening Test

• Operates on negative feedback: high TSH = hypothyroidism, low TSH = hyperthyroidism
• Changes occur early, before free T4 falls outside normal range
• Logarithmic changes make it highly sensitive

When to Test

• Symptoms of thyroid dysfunction
• Family history of thyroid disease
• Autoimmune conditions (type 1 diabetes, celiac)
• Pregnancy planning or during pregnancy
• Monitoring thyroid medication — check 6–8 weeks after dose change
• Routine screening every 5 years from age 35

Beyond TSH

• If abnormal, follow up with free T4 and thyroid antibodies (anti-TPO, TSI)