Thyroid UK
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Doctor's Best Vitamin D3

Has anyone had experience of this brand of supplement?

In Jan 2017 my vit D reading was 63.4nmol/L. I started taking Doctor's Best 5000iu vitamin D3, together with Vit K2,on & off, not sure of what dose I needed but not wanting to overdose, with a view to increasing my vitamin D in line with recommendations on here .However, in July 2017 my reading had reduced to 54.8nmol/L. At this stage my doctor prescribed Pro D3, 10,000iu every other day equating to 5,000iu daily. By Sept 2017 my reading had risen to 103.5nmol/L. As I thought this was an ok level I went back on the Doctor's Best 5,000iu every other day , equating to 2,500iu daily, expecting it to be a maintenance dose. To my surprise my reading has now decreased to 69.5nmol/L.

Am I doing something wrong, or could these particular supplements be "faulty"? I would appreciate any advice whatsoever.


12 Replies


I've been using Doctor's Best for >3 years since my vitD deficiency was corrected with ProD3. I overdid it on 5,000iu daily and vitD rose to 390. Since then I've maintained level around 100nmol/L by supplementing 5,000iu per week most of the year and 10,000iu per week during the winter.

VitD levels drop Oct-Apr when ultraviolet light is low. Try 7,500iu daily for 6 weeks to raise vitD to 100-150nmol/L then reduce to 5,000iu daily and retest during April. You may need to use 5,000iu daily as a maintenance dose Oct-Apr.


Thanks, as others also say that Doctor's Best D3 are ok I will initially increase the dose in line with your recommendation & see how it goes as I have plenty of the capsules left.


I did very well on Doctors Best D3 when I was severely deficient, going back about 2 years. I take a different brand now that I am optimal and just need a maintenance dose.

With K2-MK7, 100mcg is fine for D3 doses of up to 10,000iu D3.

Do you take Magnesium? It is an important cofactor of D3, it helps your body to use the D3

If you are Hashi's you might be best changing to an oral spray for better absorption.


Thanks for your reply. I don't take magnesium at the moment but have thought about it having seen your advice to others. I haven't bothered to ask my doctor for advice on this as she thinks a D3 reading of 69.5 is acceptable & was unaware of K2-MK7, what can I say!

Six months ago I went gluten free & since then have been getting meds for another autoimmune condition sorted so didn't want to start magnesium till I knew that these were ok.

Now may be a good time to start, but not sure which form to take. What are your thoughts on these :- Doctors Best High Absorption Magnesium Chelated - 120 x 100mg Tablets?

I look forward to hearing from you, your knowledge on these matters is amazing.


Check out the different forms here and see which will be best for you

I have always used magnesium citrate and it has helped my lifelong constipation, I use Natural Calm Original powder as you can start low and build up, plus it contains no other ingredients. I am now looking at Magnesium Taurate as well due to recent heart problems.


Thanks will check them out.


Presumably you have Hashimoto's? High antibodies

In which case our gut function is severely impaired

Try Better You vitamin D mouth spray. Comes in various strengths. Perhaps try 3000iu once daily

Retest after 2 months. If not enough increase to twice daily

What about ferritin, folate and B12 levels

Are you strictly gluten free if you have Hashimoto's

Are you on enough Levothyroxine to bring TSH down to around one and FT4 towards top of range and FT3 at least half way in range

Dr Toft, past president of the British Thyroid Association and leading endocrinologist, states in Pulse Magazine,

"The appropriate dose of levothyroxine is that which restores euthyroidism and serum TSH to the lower part of the reference range - 0.2-0.5mU/l.

In this case, free thyroxine is likely to be in the upper part of its reference range or even slightly elevated – 18-22pmol/l.

Most patients will feel well in that circumstance. But some need a higher dose of levothyroxine to suppress serum TSH and then the serum-free T4 concentration will be elevated at around 24-28pmol/l.

This 'exogenous subclinical hyperthyroidism' is not dangerous as long as serum T3 is unequivocally normal – that is, serum total around T3 1.7nmol/l (reference range 1.0-2.2nmol/l)."

You can obtain a copy of the articles from Thyroid UK email print it and highlight question 6 to show your doctor

please email Dionne:

Prof Toft - article just published now saying T3 is likely essential for many


Thanks for your advice.

I do have Hashimoto's , together with other autoimmune diseases.

I have been gluten free for 6 months.

My latest thyroid test, done last week, shows TSH 0.83mu/l (0.35-4.94) Free T4 18.4mu/l (9.0-19.00). On 125 levothyroxine daily.

I also have B12 injections every 10 weeks.

Will also be trying recommendations of Clutter & SeasideSusie.


Essential to have good levels of folate, ferritin and B12 plus vitamin D

You might consider supplementing B12 sublingual lozenges between injections.

Good vitamin B complex daily too

But remember to stop any supplements with biotin in 3-5 days before any blood tests as biotin can interfere with test mechanism and cause false results

you really need to know your FT3 result

NHS often refuses to test (says its too expensive, apparently only costs 92p)

As a result you will almost certainly have to get private test done

Via Medichecks or Blue Horizon

Your TSH and FT4 are about perfect, If FT3 is still low , then like many with Hashimoto's you may need the addition of small dose of T3.

Thyroid UK have list of recommended thyroid specialists. some are T3 friendly




I had exactly the same as you. My blood test result was 80nmol/L after I had been taking 1000iu vitamin D3 (Vitabiotics Ultra). I then started taking Doctor's Best 5000iu vitamin D3 daily and after 3 months the results showed 72nmol/L so it had come down. I've now started using the Better You vitamin D mouth spray to see if this will increase the levels.

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In a strange way it's good to know it's not just me!

Others have recommended the mouth spray so will check that out.

Will be interesting to see how it works for you.

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