Hey Judy,

Good question and you're correct as explained in the following discussion.  Obtaining 25(OH)D lab tests can be problematic at times and part of this has to do with the physician's understanding of vitamin D3 and its physiological roles. 

The following chart developed by Dr. Robert Heaney, MD, an acknowledged vitamin D3 expert, illustrates the "Average" 25(OH)D time course response to increasing doses of vitamin D3 from none to 10,000 IU/day.  The color bands represent data from the online survey of 187 CHers taking the anti-inflammatory regimen at a dose of 10,000 IU/day vitamin D3 and experiencing a favorable CH response.  As you can see from the green color band, the therapeutic serum concentrations of 25(OH)D range from 60 to 100 ng/mL... actually closer to 110 ng/mL.

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The next chart, also developed by Dr. Heaney, illustrates the time course serum concentrations of vitamin D3 and 25(OH)D following a single oral dose of 100,000 IU vitamin D3.  The first curve with the large amplitude spike is the serum vitamin D3 concentration.  It reaches T_max (time to reach the maximum serum concentration after dose) at day two then drops rapidly in serum concentration to zero or baseline at day 14.

The second smaller amplitude curve represents the time course serum concentration of 25(OH)D, a.k.a., 25-Hydroxyvitamin D3 or cacidiol, the first metabolite of vitamin D3.  As you can see this curve reaches Tmax at approximately day seven after dose and then drops slowly to baseline at 4 months.

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My understanding of the pharmacokinetics of vitamin D3 (What the body does to vitamin D3), tells me that a lab test of serum 25(OH)D should be relatively accurate 2 weeks after any given oral dose of vitamin D3.  I say "relatively accurate" because there are several factors that can influence this serum concentration with some making its concentration higher and most others making its serum concentration lower.

The 3-month time interval used by your doctor for the next lab test of 25(OH)D would tend to be a more accurate measurement owing to more time to reach a stable equilibrium.  That said, a lab test for 25(OH)D taken at 3 months is still subject to the same factors as a measurement at one or two weeks.

Ultimately, the best indication you've reached a therapeutic dose of vitamin D3 is a complete cessation of CH symptoms or a significant reduction in the frequency, intensity or duration of your CH... i.e., going from an average of 3 CH a day down to 3 CH/week...  I think you'll agree that would be a significant change...

The other two lab tests that are even more important than serum 25(OH)D are serum concentrations of calcium and PTH.  What we all want to see as results of these two lab tests is our serum total calcium within it's normal reference range of 8.5 to 10.5 mg/dL and a serum PTH concentration near the lower limit of its normal reference range of 17 to 70 pg/mL.

As long as our serum calcium stays within its normal reference range, there's no vitamin D3 intoxication regardless of how high our serum concentration of 25(OH)D goes above 100 ng/mL.

The reason we want to see our PTH at the low end of its normal reference range is simple once you understand its role in calcium homeostasis.  When the calcium serum concentration drops towards the low end of its normal reference range, chemo-receptors in the parathyroid glands sense this drop so start producing PTH.  This hormone signals the kidneys to produce more of the enzyme 1α-hydroxylase which in turn adds an [OH] radical to the 1st Position on the 25-Hydroxyvitamin D3 molecule to make 1,25(OH)2D3, a.k.a, calcitriol, the active serum metabolite of vitamin D3.  It's serum 1,25(OH)2D3 that to goes to the gut to pull out calcium and push it into the bloodstream.

Accordingly, as long as the serum concentration of PTH remains low, we're not adding more calcium to the bloodstream.  This keeps us from experiencing vitamin D3 intoxication as indicated by a serum calcium concentration above its normal reference range.

(Edited to add the following)

If you've been at a stable maintenance dose for 6 months, a lab test for serum 25(OH)D, total calcium and PTH once a year is adequate.

If you're just getting started on this regimen or recently started a vitamin D3 loading schedule in response to falling out of remission due to an infection, surgery, trauma or an allergic reaction, obtaining labs for 25(OH)D, total calcium and PTH a month after returning to a maintenance dose would be appropriate.

'Hope this answers the mail...

Take care and please keep us posted.

V/R, Batch