Hi Carol
Chemo seems to be a fairly unique experience for every man who has it for PCa, there are some common side effects, some men literally go through 6 -10 cycles of docetaxel chemo with next to no side effects at all others who are less fortunate get most or all of them at one stage or another.
I am no expert but this is what I think might be the cause and it is definitely worth a follow up.
The extreme fatigue is quite common, in many cases it is caused by a temporary drop in Hb (Haemaglobin) which causes anaemia. Your Husband should have been given a red chemo diary/record book which gives a guide to the treatment, things to look out for and when to be worried enough to call for help. One of those factors is tiredness and activity levels. From what you have said I think you should be calling your Husband's medical team for guidance, this is not a major problem or necessarily a setback. However, Your Husband may need some extra medication or treatment to improve his energy levels.
This A5 sized red book also has a record page for each cycle of chemo. There is a section that the chemo nurse completes before chemo is given, it should tell you the Hb level along with other blood results.
Hb is normally reported with a decimal point but sometimes as a whole figure, for example 13.5 or 135
the normal range for a healthy adult male is 11.5 - 18.0 or 115 -180
If the figures are below that and energy levels are low with extreme fatigue, then sometimes,the Oncology team decide that a blood transfusion might help significantly. I was very naieve when I was first told that Mick needed a blood transfusion thinking it was a really big and dramatic thing. It is actually quite straight forward and not at all the scary thing I first thought. In Mick's case after cycle 3 his Hb dropped to 8.8 and at that point he was like your Husband, wiped out tired all the time. He had the first transfusion of 3 units of blood next day and perked up almost immediately. He had 3 more transfusions in between cycles 4, 6 and 7 and in every case they made a big difference. His Hb went up to 11.1 and although it dipped towards the end of the 3 week cycle it recovered sufficiently every time so that chemo could continue.
Once chemo ends if things are still improving and the cancer is being pegged back the Hb should slowly right itself and the fatigue and low energy levels should steadily improve.
The reduction in PSA from 2300 to 535 is good, big drops initially then a little slower as the starting number itself gets lower.
I wish you all the very best
xx
Mo