Radar
I am really surprised that your GP seems to be somewhat dsmissive of your low Hb and weight loss. Also that anyone would just say you were low in iron.
It is fair to say that anaemia is commonly associated with low iron levels, if that is the cause then being treated with iron supplements or a diet rich on iron with dark green vegetables such as spinach, kale or spring greens will normally correct things. However, from what you have said, your anaemia is not due to lack of iron. It could be related to your PCa but as Lyn has said that is more common in advanced disease or as a side effect of treatments like chemo.
Low Hb can be caused by many illnesses a common cause is blood loss from a trauma, or slow internal bleeding from an ulcer or a chronic condition such as Colitis. If it is low end of the scale as in within the acceptable range (for an adult male 13.0 to 17.0 g/dl sometimes expressed as 130 to 170 although different trusts use a slightly varied scale) then its unlikely treatment would be necessary or offered, but if it is lower than that or accompanied by another change such as weight loss, and yours is quite quick and high, then finding the underlying cause would be,in my untrained view, more important.
If this is a worry for you then I would return to your GP or call your consultant's secretary and ask if you can speak to one of the specialist team about it.
In any event any dramatic or prolonged unplanned weight loss needs investigating.
I hope you can get some help with this and some answers/action asap.
Best wishes
xx
Mo