TSH (Thyroid Stimulating Hormone)

The master thyroid regulator — the first test to order for thyroid function.

What it is

TSH is produced by the pituitary gland and stimulates the thyroid to produce T3 and T4 hormones. It is the most sensitive indicator of thyroid function — it rises before T4 falls in early hypothyroidism and falls before T4 rises in early hyperthyroidism.

Why it matters

Several immunotherapy drugs (especially checkpoint inhibitors) cause immune-mediated thyroiditis as a side effect. Thyroid dysfunction — which causes fatigue, weight changes, mood disturbances, and cognitive symptoms — can be mistaken for other treatment side effects or disease progression. TSH is the screening test to rule this out.

How to test

Blood test. No fasting required. If abnormal, order Free T4 and Free T3.

This information is for educational purposes only. Always discuss testing and interpretation of results with your care team.

Optimal range

Below 0.4 mIU/L

low — hyperthyroid (or overmedicated)

0.4–4.0 mIU/L

normal range

Above 4.0 mIU/L

elevated — hypothyroid — affects energy, metabolism

How often

Every 6–12 months baseline. Every 6–8 weeks if on immunotherapy.