What is TSH?
TSH responds to changes in thyroid hormones and is therefore a standard starting point for thyroid assessment. If TSH is abnormal, Free T4 is often added to clarify the picture. Interpretation is context-dependent and is best viewed in trends.
Why is TSH relevant?
TSH is the regulatory hormone that drives the thyroid — an elevated value means the brain is amplifying the signal because circulating thyroid hormone is low, a suppressed value means the opposite. For symptoms like fatigue, cold intolerance, weight shifts, or mood changes, TSH is almost always the first marker you check, because the system is so sensitive that it often reveals thyroid issues early.
How to read TSH in context
TSH is interpreted on its own but sharpens with Free T4 (and sometimes Free T3) alongside it — without them, you miss the distinction between primary and secondary issues. Time of day matters: TSH varies on a 24-hour rhythm and is typically higher in the morning. For people on supplementation or medication, TSH is re-tested at least 6–8 weeks after a dose change to let the system reach steady state.