Thorns are not uncommon on citrus, especially trees that are relatively young. Check to make sure the thorny branches are not emerging from the base of the tree, below the graft union. That would indicate that they are rootstock shoots which should be cut back to the point of attachment. If for some reason the grafted top part died then the entire tree could be rootstock growth but this is not the most likely situation. The only way to tell if it is a grapefruit is to wait for it to produce fruit so the type of citrus can be identified.

Thank you for reply. I bought another Grapefruit tree recently and compared the leaves of each, (they look the  same) and attach a photo of the tree in question. The leaves have a large portion, then a smaller portion closer to the stem. Does the photo look like a Grapefruit tree - not root stock? Is it normal for a Grapefruit tree not to put out blossoms for 5-6 years? I'd like to cut it down if it's not GF tree.  Thanks again.

Based on your submitted picture, I would rule out the possibility that you are waiting on rootstock of some other citrus type to bloom or bear fruit. The foliage is consistent with grapefruit, but I cannot tell from the foliage whether this plant even has a rootstock. It is not common for a citrus tree grafted to grapefruit to not flower. Usually they flower the first year you have them--certainly the second. However, if the grapefruit was grown from seed and not grafted, then it could take 5-6 years (or longer) to begin flowering. You should be able to determine whether there is a rootstock by observing the trunk 4-5 inches above ground level and looking for a slight bark change, even presence of suckers below the graft having a different foliage type. If not, then you may be dealing with seedling/juvenile grapefruit that will eventually flower.

I also notice from your picture that your foliage is very pale green--indicative of nitrogen deficiency. I also see leaves that have been bronzed by red spider mites. Flowering and fruit set of citrus trees can be impaired by poor nutrition and stress from foliage pests. They also need well-drained soils, good watering and protection of the foliage from freeze damage..
I recommend that you increase the nitrogen fertilization for this tree, and either conduct a soil test to determine other nutrient needs (P,K, Mg, etc) or apply your nitrogen as a complete fertilizer (10-10-10, or other) with micronutrients.
A 5-6 year old citrus tree should receive from 0.33 to 0.50 lbs actual nitrogen per tree, so with 10-10-10 fertilizer, that would be 3.5 to 5 full pounds of fertilizer per tree per year.
Soil testing information can be found at http://soiltesting.tamu.edu
Our horticultural information is housed at http://aggie-horticulture.tamu.edu

Thank you again for your response. You saved this tree. I was going to back my truck up to the tree and tear it out. There is no indication that there was ever a graft above ground level. There was never a sucker from the root stock below which often happens.I'll let it live and follow your fertilizing suggestions.  Thanks