TSH, T3, T4: A Beginner's Guide to Reading Your Thyroid Panel
TSH gets all the attention, but it's a signal from the pituitary gland, not a thyroid hormone itself. Here's how TSH, free T3 and free T4 fit together.
If you've just been handed a thyroid panel for the first time, the abbreviations alone can be overwhelming. Here's the short version of how the three main numbers relate.
TSH is an instruction, not a hormone
TSH (Thyroid Stimulating Hormone) is made by the pituitary gland, not the thyroid — it's essentially a signal telling the thyroid how much hormone to produce. That's why the logic runs backwards from intuition: high TSH usually means the pituitary is asking loudly because thyroid output is running low, and low TSH usually means the opposite.
T4 and T3 are the actual hormones
Free T4 is the main hormone your thyroid releases; Free T3 is the more active form, mostly converted from T4 in other tissues. "Free" matters because most circulating hormone is bound to protein and inactive — the free fraction is what's actually available for your body to use.
Why one TSH reading rarely settles anything
TSH follows a daily rhythm, shifts with acute illness, and is affected by several common medications. A single borderline TSH with a normal free T4 is usually a reason to retest in a few weeks, not an immediate diagnosis.
What to track if you're already being monitored
If you're on thyroid medication, the practical question isn't "is this normal" but "is this stable at my current dose." That needs TSH and free T4 tracked together across repeat tests — ideally at a consistent time of day, since TSH itself varies through the day.
This is general health information, not medical advice. Always talk to a qualified healthcare professional about your own results.
