I dieted and lost 30 lbs, blood pressure has dropped to average 116/69 and my gfr dropped to 58. i'm 70. should i be concerned over the gfr?

Probably not: Probably not. The bottom of normal (for most labs is 60) so you should be ok. Talk to your doctor.

Maybe: You should talk with your doctor. It really depends on whether you have diabetes or hypertension. This drop can signal early kidney disease. It should be repeated and monitored.

Depends: A GFR over 60 is considered normal. However, if you have had a recent significant drop in your gfr, that should be evaluated further. If your GFR is stable at 58 it is most likely ok. There are many other lab values (creatinine/potassium)and clinical values (water weight/respiratory status) that are evaluated before an assessment is made over kidney function.

Monitor eGFR: Nowadays estimated gfrs are too sensitive. You may get a better number with a 24 hour collection of urine for creatinine clearance. Otherwise monitor your egfr. Protein in the diet may falsely increase creatinine. Also if you where put on aceinhibitors (lisinopril, enalapril, Captopril or ....Pril) that may increase your creatinine if you have poor circulation to the kidney. Monitor your egfr.

EGFR: It's good you lost weight and lower your blood pressure. Why egfr drops is probably not associated to that. Estimated GFR is just an estimate. Usually diets rich in protein intake increase the creatinine giving a false reading of kidney function. Gfr may drop because damage from hypertension, diabetes or poor arterial circulation to the kidneys or other kidney disease.