Despite taking 4,000 to 8,000 units per day for the past few months.
National Institute for Health and Care Excellence
NICE
Do not routinely test for vitamin D deficiency in people who are asymptomatic.
https://cks.nice.org.uk/topics/vitamin-d-deficiency-in-adults/diagnosis/diagnosis/
Check the vitamin D level by measuring serum 25-hydroxyvitamin D (25[OH]D) if a person has:
Musculoskeletal symptoms
Suspected osteomalacia.
Chronic widespread pain with other features of osteomalacia (such as proximal muscle weakness).
Suspected bone disease that may be improved with vitamin D treatment, such as osteomalacia or osteoporosis.
Known bone disease, where correction of vitamin D deficiency is needed prior to specific treatment, such as:
Prior to Paget’s disease treatment with a bisphosphonate.
Note: asymptomatic people at higher risk of vitamin D deficiency do not need routine testing for vitamin D deficiency,
but should be advised on the need for maintenance dose vitamin D supplementation.
Source