What will happen when humans are injected with plant growth hormones
I will only give an outline based only on auxins otherwise the question may be deemed too long.I hope this helps a bit.

The first auxin was isolated from the urine of pellagra patients.

Most natural auxins are indole derivatives which are the intermediates or final products of tryptophan metabolism.Both vitamin B3 and auxins are consequences of tryptophan metabolism. [source]

POSSIBLITY 1-

Thus, auxin injected may be eliminated through urine.

We already understand that substances that are excreted via urine are either toxic to the body or above threshold.Perhaps this idea was harnessed by researchers to construct researches which proved that auxins have anti-cancer properties. 


  given that the chemical structure of plant and human auxin is identical and that cytokinin IPA is found in human cells bound to tRNA assisting protein expression, presumably supplied by the consumption of vegetables in the diet, and that we observe a positive effect of plant hormones on pluripotency and key anticancer gene expression levels in mammalian cells [1],[2]


POSSIBILITY 2-

In proper dosage may be utilised for its anticancer properties.

Cytochrome and a series of enzyme system in liver cells are known to detoxify indole

Protein putrefaction in the large intestine results in formation of indole.This indole is carried by the portal system to the liver where it is detoxified to indican  with the help of the cytochrome P450 isoenzyme system and consequently eliminated via urine.

POSSIBILITY 3-

Thus auxins (being indole derivatives and moderately hydrophilic) may be detoxified in the liver and then excreted.
REFERENCE:[1],[2],[3]