The condition that you describe is urticaria
that is due to mast cell release of histamine
and can have several triggers such as heat, cold, pressure, stress and infections.
Autoimmune urticaria (thyroid problems or otherwise) is now a major form of this condition and therefore you need tests such as CBC, ESR, LFT, TSH fT4 fT3, TPO antibodies
, ANA and must be on long acting antihistamines such as Fexofenadine
for 6 weeks (minimum) or 3 months if still getting eruptions more than 2 times per week to control this condition.
80% of sufferers do NOT have allergy, so allergy testing is usually unhelpful.