Glomerular filtration rate (GFR) measures the ability of the kidneys to remove (clear) creatinine from the blood. Kidney disease can cause a decline in the (GFR) and result in excess creatinine in the blood.
Apart from the increase in creatinine with a decline in GFR, creatinine can vary over time and can is affected by certain disorders, not limited to thyroid disease and medications such as diuretics.