TSH

What it is

TSH (thyroid-stimulating hormone) isn't made by your thyroid at all — it's made by the pituitary, a small gland at the base of the brain that acts like a thermostat, turning the dial up when it senses too little thyroid hormone and down when it senses too much. Because of that, TSH reads high when thyroid output is low and low when output is high, which is why it's usually reviewed alongside Free T4 and sometimes Free T3 rather than alone. It naturally varies through the day (generally higher overnight and in the early morning) and can shift temporarily during illness, pregnancy, or with certain medications, and reference ranges differ between labs and assays — so a single borderline value is often repeated before it's interpreted.

Typical range

Reference ranges vary between laboratories, and what is normal for you depends on your own history. Your own lab’s range is the one printed on your report.

Worth asking your clinician

• Given the time of day my blood was drawn and how I've been feeling, how much weight should we put on this TSH result?
• Would it help to look at my TSH alongside Free T4 or Free T3, or to repeat it before drawing any conclusions?
• Is there anything in my history or medications that could be affecting this number?