Tathagatagarbha and dhammakaya Can we consider 'tathagatagarbha' and 'dhammakaya' as synonyms? If not, what is the difference between those words.
No, they are not synonyms. They are mutually exclusive: If it is 'tathagatagarbha', then it is not 'dhammakaya'. If it is 'dhammakaya', it is not 'tathagatagarbha'.

Tathagatagarbha is 'suchness with defilement', which is the emptiness of the minds of sentient beings.

Dharmakaya is 'suchness without defilement', which is the emptiness of the omniscient mind of a buddha. (more specifically any of the 'nature truth body', according to Prasangika). Although they are undifferenciated in nature (pure by nature), Dharmakaya and Tathagatagarbha are not the same.

As Gyaltsab-Je states in his commentary to Maitreya's Sublime Continuum
differentiating the Lineage of the Three Jewels:


  Suchness with defilement is the basic element not liberated from the
  obscurations of mental afflictions, and is known as tathagata
  essence." By nature it is pure and is the ultimate truth. The term
  "tathagata essence" is not used anywhere by either the Tathagata
  Essence Sutra or this treatise and its commentary, which explain the
  thinking of that sutra, to teach the  twofold purity [i.e. the
  dharmakaya endowed with the twofold purity]. Therefore, to apply the
  ultimate dharmakaya to "tathagata essence" is a mere fabrication