My free t4 was highlighted as low (11.1 pmol/l). but my tsa was normal (2.10 munit/l). so my doctor said not to be worried. im also low in vitamin d, phosphate and iron. should i be concerned about the t4?

What is the reason for checking thyroid function test at your age? The range of free T4 varies between laboratories, some put 10.3 pmol/L at the lower limit. If you have no symptoms & this is just a health check then you have nothing to worry. However, if you are symptomatic (weight gain, irregular cycle, etc) then further step needs to be done. Vit D, PO4, iron deficiency are unrelated problems.