TSH (thyroid-stimulating hormone)

A high TSH most often means an underactive thyroid (hypothyroidism); a low TSH usually means an overactive one (hyperthyroidism). The ATA/AACE guideline distinguishes a subclinical zone, where TSH is above the reference range but free T4 is still normal, from overt disease. Above 10 mIU/L the guideline notes increased risk of heart failure and cardiovascular mortality and says treatment should be considered; between the upper reference limit and 10, treatment is individualized around symptoms, TPO antibodies and cardiovascular risk.

This is the most asked question about TSH and it deserves a direct answer. A TSH number on its own is not what decides whether a situation is urgent, and the thyroid emergencies are not diagnosed from it.

The emergencies are a clinical picture, not a lab value. A 2026 joint consensus statement on thyroid storm, from the European and British thyroid associations, is explicit: "There is no serum thyroid hormone level or specific test that delineates severe thyrotoxicosis from thyroid storm." The same statement adds that "TSH should not be used to guide early treatment response." The score clinicians use alongside that judgment, Burch-Wartofsky, is built from temperature, central nervous system disturbance, gastrointestinal and liver dysfunction, tachycardia, congestive heart failure, atrial fibrillation, and whether there was a precipitating event. No thyroid blood test contributes a single point to it.

A very low reading is usually a diagnosis rather than an alarm. The American Thyroid Association's 2016 hyperthyroidism guideline defines overt hyperthyroidism as a subnormal TSH, typically undetectable on a modern assay, together with raised T3 or free T4. That tells a clinician what is happening. It does not, by itself, say how unwell you are.

The same caution applies in the other direction. The association's 2014 hypothyroidism guideline, writing about myxedema coma, says "It must be remembered that the degree of TSH elevation may not be an accurate indicator of the severity of the hypothyroidism based on the wide range of serum TSH values seen in myxedema coma, possibly due to variable suppression of the hypothalamic-pituitary axis by the illness." Its instruction for that emergency is not to wait for the laboratory at all: "Therapy can and should be instituted based on clinical suspicion alone and should not be delayed while waiting for blood test results."

So what should you act on? The features those scores are built from. Fever, a racing or irregular heartbeat, breathlessness or swelling, persistent vomiting or diarrhea, yellowing of the skin or eyes, agitation, confusion, or unusual drowsiness. In the opposite direction, being very cold, very slow, or confused. If any of those are present, seek care now and do not wait for a repeat test. If none are, a TSH result however far outside the reference interval is a conversation with your clinician, usually within days, rather than an emergency.

If your report is flagged. A flag is your own laboratory's decision, and that is federal law rather than an accident. CLIA requires a laboratory's procedure manual to set out its own "Imminently life-threatening test results, or panic or alert values", and treats reference intervals as the ones "as determined by the laboratory performing the tests". The regulation never says what those values are: the words panic, critical value and alert value appear nowhere in its definitions at 42 CFR 493.2. So there is no national threshold to look up, and the number that flagged your result belongs to the lab that ran it. What the law does require is that the laboratory "must immediately alert the individual or entity requesting the test and, if applicable, the individual responsible for using the test results" when a result is critical. Neither of those is you. Your right to the report itself comes from somewhere else. HIPAA gives you a right of access to your own test reports, and the same 2014 rule that added this CLIA paragraph removed the exception that had kept laboratories out of it. You can get the result. Nobody is obliged to phone you about it, which is why a flag can land with nobody having called. The NIH's own MedlinePlus puts the reading rule plainly: "You can't compare your results to reference ranges that you might find elsewhere." Use the range on your own report, and ask your clinician what threshold their lab uses. If any of the features above are present, none of this matters, go now.