A 32 year old female presented with complaints of fatigue and breathlessness since 1 month. Patient had gained weight in the past 1 month, and had intolerance to cold and complained of hair loss. On enquiry, patient said she had delivered a child 3 years back, and was unable to breastfeed due to lack of milk. Patient also complained of irregular cycles since 1 year. What test would be most appropriate to determine the cause?