I previously posted how:
As of December 19, 2014, I have been targeting a Vitamin D-3 intake, in pill form, of 7,000 IU per day, for over two months. My other sources of Vitamin D-3 ought to be negligible (little or no exposure to sun). My D25OH test results have been as follows:

Sample DateResultAverage Daily Change
October 8, 201427.15 ng/mL/
November 12, 201449.71 ng/mL+0.64 ng/mL/day
December 12, 201458.71 ng/mL+0.30 ng/mL/day

I have three hypotheses for why the average daily change has dropped off towards the third test:
  1. Body adjusts. Perhaps the body makes use of more Vitamin D-3 when more is available in long-term storage. This would be excellent news - it would mean a person can continue to take a consistent amount without worrying too much about overshooting the 60 - 80 ng/mL range. As long as the intake is within reason, the body will use more if more is available.
  2. Pill inconsistency. Perhaps reported pill content is unreliable. All my intake of Vitamin D-3 has been in the form of pills from GNC, but the pills have come in a variety of different form factors. I began with 7 x 1,000 IU per day; then I had to switch to 17 x 400 IU because a larger pill was not available locally; then I switched to 1 x 5,000 IU + 2 x 1,000 IU when Jana returned from a trip to the US, and brought a supply from GNC there. If pill content is inconsistent and unreliable, that would be bad news; staying in the 60 - 80 ng/mL range would require frequent blood testing.
  3. Daily variation. Perhaps there are daily fluctuations in metabolism that might cause results to vary by a large amount, depending on when the sample is taken.
I'm leaning towards either the first or the second hypothesis. Of the two, I'm very much hoping for the first. For now, I'm continuing to take 7,000 IU per day. In January, I plan to do a 4th blood test. I'll be able to take my daily 7,000 IU in the same form factor as between the 2nd and 3rd test, so my intake should be consistent. The results of the fourth test in January may then provide further evidence. If average daily change drops further, that will speak in favor of hypothesis 1 - the body is using more as it gets more. If average daily change remains the same as between 2nd and 3rd test, that will be in favor of the second hypothesis - pill content may be unreliable. If daily change returns to what it was between 1st and 2nd test, that would suggest hypothesis 3 - large daily variation.

I should note that my average mood and enthusiasm have improved consistently with D25OH blood level. There are differences from day to day, but I now feel much more enthusiasm for work and general activity than e.g. last month. Since this is not a blind study, it continues to be impossible to rule out placebo, but the improvement has been consistent with my long-term level climbing up.

Edit - February 15, 2015:

I have continued to take Vitamin D-3 in the same form factor at 7,000 IU per day, and got tested again. Updated table with latest results:

Sample DateResultAverage Daily Change
October 8, 201427.15 ng/mL/
November 12, 201449.71 ng/mL+0.64 ng/mL/day
December 12, 201458.71 ng/mL+0.30 ng/mL/day
February 13, 201571.35 ng/mL+0.20 ng/mL/day

This seems to support hypothesis 1 - the body adjusts, which is good news.

Edit - May 15, 2015:

I have continued to take Vitamin D-3 from the same GNC bottle, one pill of 5,000 IU per day since my last update in February. New latest results:

Sample DateResultAverage Daily Change
October 8, 201427.15 ng/mL/
November 12, 201449.71 ng/mL+0.64 ng/mL/day
December 12, 201458.71 ng/mL+0.30 ng/mL/day
February 13, 201571.35 ng/mL+0.20 ng/mL/day
May 14, 201567.68 ng/mL-0.04 ng/mL/day

Amazingly, 5,000 IU per day was not sufficient to maintain a stable level of 70-75, and I have dipped a bit to 67.68. I did not expect that.

I will continue at 6,000 IU per day for a few months.