- Heavy metal poison from eating raw fish for decades in a cave in a goblin mine
- Vitamin B12 and iron deficiency from an all-fish diet
- Exclusion of said deficiencies because Gollum augmented his diet with occasional goblin; exclusion of said deficiencies because of lack of associated peripheral neuropathy
- Heavy metal / mercury / radiation poisoning from the One Ring
- Exclusion of heavy metal poisoning because of maintained high mobility and strength
- Addiction (to the Ring)
- Obsessive-compulsive disorder (ring related), comorbid with antisocial personality disorder
- Traditional Chinese Medicine diagnosis of Kidney Yin deficiency and liver-yang excess
- It's just symptoms of extreme old age
- Symbiotic parasite infection from the Ring
- Borderline personality disorder
- Chronic bacterial/fungal infection
- Psychopathy (murdered Deagol on sight of the Ring)
Gollum himself weighs in in the comments:
Nasty psychiatrissstss! Hates them, my precious! They locks uss up in padded cell! They makes uss look at inkblotsss! Tricksy, sly inkblotsss! Nasty Elvish pills burnsss our throat!
Yesss We Hatesss themsss Evil oness yess my preciousss we hatess themsss
But They Helpsss us!
No they hurtsss usss, hurtsss usss sore!