While I agree that it is reasonable to expect supplements to contain the specified amount of the claimed active ingredient, we also have to remember that RX prescription vitamin D2 Ergocalciferol is at best only one third as good as Over the Counter Cholecalciferol. So If you buy 50,000iu of D3 and it contains only have the stated amount you are still getting more active vitamin D than if you used vitamin D on prescription from your doctor.
In the UK there has been a long history of prescription vitamin D2 suffering from batch variability so it's probably true that ergocalciferol is less stable than cholecalciferol and there is also the fact that no one can predict if you are one of those people whose bodies don't recognize D2 at all so it does no good (or harm) however much you take, I've had reports of people taking 200,000iu daily of Ergocaciferol with absolutely no change in 25(OH)D.
While there is a great range of response to different doses (and different makes of D3) we can see from the chart below that EFFECTIVE intakes generally raise status to 60ng/ml.
The only way you can be sure what you are doing is working for you is to
get regular 25(OH)D tested.
Don't forget that in the same way you may not be safe relying on the potency of vitamin D supplements so you also cannot rely on the amount of UVB that reaches the ground from sunlight. It all depends on the amount of ozone in the atmosphere where you live. In the country, providing you are not under a flight path or in the lee of a factory then your skin will make more D3 because there is a higher UVB ratio than someone at the same latitude spending the same amount of time at the same time of day but who lives in town.
Bear in mind the response to D3 also varies with the fat content of the meal you consume with your D3. If you take it with the largest meal of the day and that meal has a good fat content ier butter/coconut oil/olive oil then it could be 50% more effective than the same D3 taken without food or with a low/no fat meal. It's worth always getting your D3 in an oil based gel to ensure it's starting off in fat.
These Country Life, Vitamin D3, 5,000 IU, certainly work for me and my partner as both of us have 25(OH)D's in above 64ng/ml.
We also make sure we get plenty of sun exposure. I think there will be other benefits to health from sun exposure apart from the Vitamin D3 route. So by taking a basic amount to cover daily vitamin D3 needs and using sun exposure to raise the stored reserves of D3 you can be sure you won't get to toxicity status. (the amount of vitamin D your skin makes is regulated according to your 25(OH)D status. People with low vitamin D levels make more D3 given sun exposure than a person with a high vitamin D level spending the same time in the same sun at the same latitude. The higher your vitamin D status gets, the less your skin makes from sun exposure. Bear in mind it's the reverse for cholesterol. The higher your cholesterol level the more D3 your skin can produce the lower your cholesterol level the LESS vitamin D can be made. (UVB acts on cholesterol to convert it to D3)