Tertiles calculations

#1
I need some help with statistics calculations.
I have a list of variables as

sample#id weight
id1-00 | value_1_00 |
id1-01 | value_1_01 |
id2-00 | value_2_00 |
id2-01 | value_2_01 |
...
idn-00 | value_n_00 |
idn-01 | value_n_01 |

where, "id" identifies a patient. For each of the patients i have a value of weight at baseline (00) and after one year of treatment with diet (01).
What I need to do is to differentiate my data in order to have three categories (tertiles) where I can discriminate patients which gain weight (weight after one year of treatment, > 5Kg than baseline value), patients which maintain weight (weight after one year of treatment, not > or < than 5 kg than baseline value), patients which lost weight (weight after one year of treatment < 5kg from baseline value).
Can anyone help me?
Many thanks