Thyroid storm is a clinical diagnosis
Despite difficulty in the fact that many patients have vague and undifferentiated features the urgency of the situation may not allow time for laboratory confirmation. If you suspect it, treat it.
The presentation may be deceptive
The peer reviewer goes so far as to say:
The diagnosis of thyrotoxicosis can be subtle and will absolutely be missed by the clinician who has not considered it on his or her differential.
The Burch-Wartofsky score may be helpful
It's a clinical tool for evaluation of the probability of thyroid storm independent of lab results and can be accessed here.
Order and timing of medications
Wait at least an hour after giving the thionamide before giving iodine to avoid exacerbation of storm as a consequence of iodine.
Understand the relationship to infection
Infection is one of several known stressors that can convert ordinary thyrotoxicosis into storm. So in the patient presenting with storm evaluate for underlying infection and have a low threshold for starting antibiotics. It works in the other direction. Since the manifestations of sepsis and storm overlap considerably, avoid premature closure and consider the thyroid in patients presenting as sepsis.