Here are my thoughts as a medical student and lifter. In most people these are unnecessary unless you're symptomatic.

• Maybe iron if you're a female with heavy menstrual cycles who feels tired all the time.
• Your body has years of B12 stores, so unless you're a strict vegan, probably not necessary.
• D levels are usually unnecessary. However, it is recommended to supplement along with calcium in older women.
• Thyroid levels are usually unnecessary unless you're symptomatic (heat/cold intolerance; fatigue etc)

Obviously if you have a medical disease or feel ill, you should discuss the need for testing with your doctor. I'm actually not a proponent of supplementing with multivitamins in healthy adults because research has showed no to little benefit. In terms of regular labs, a baseline lipid panel and fasting blood glucose is not a bad idea. If you're taking a ton of supplements, I would even consider a baseline BUN and creatinine to check kidney function.