It used to be common practice to identify abbreviations by means of full stops.
However, modern practice is to omit the full stops.
Abbreviations which represent the name of organisations should (almost) always be in capital letters.
You should note that there are exceptions.
Certain abbreviations are always shown in lower case letters.
Others are a mixture of upper case (capitals) and lower case letters.
Dr or Dr.
Mrs (in UK) or Mrs. (in USA)
Ltd or Ltd.
When in doubt, refer to a modern (online?) dictionary.
Introduction - Capital letters - Full stops - Question Marks - Exclamation marks - Abbreviations - Contractions - Commas - Speech marks (Quotation marks - Inverted comas) - Possessive apostrophes - Colons - Semi-colons - Brackets - Hyphens - Dashes - Obliques (slashes)