Redness of the hands and fingers can be attributed to a disorder in the control of vascular tonus (dilatation and constriction). Vascular tonus, in turn, is controlled by the neurovegetative system (sympaticus and parasympaticus). These vegetative vascular reactions could be due to some kind of disturbance in the neuron-vegetative reflexes. It doesn't seem likely, however, that this particular skin redness is related with your pre-existing diseases (Diabetes, Hypertension, Hypothyroidism). You might try asking your GP (general practitioner) for a reference to a specialist who might help you diagnose and treat this condition.