Are thyroid levels meant to fluctuate?

To check a thyroid's function, one needs to do more than a T3 measurement.  Usually most docs if they are only going to do one test initially would check the TSH.  When any test shows abnormality, then more tests are usually required to see why a thyroid is not functioning normally.  While test results may vary, they usually vary within a range that is normal, i.e. between the lowest normal value and the highest normal value.  If they are out of that range, the value is considered abnormal.  Reasons thyroid glands do not function normally are often due to autoimmune diseases like Graves disease or Hashimoto's (some people have both of these at the same time).  Autoimmune antibodies produced by the immune system are attacking either the TSH receptors in the pituitary gland or in the case of Hashimoto's attacking the thyroid gland directly causing the gland to produce too much thyroid hormone.  Other causes besides autoimmune diseases are viral illnesses that cause thyroiditis during their course but will subside, after pregnancy when the Endocrine system is undergoing great hormonal changes or when the thyroid gland has a lot of thyroid nodules.  Most nodules are benign but some can be malignant and there are tests to rule this out. 

Hi james22918, research has shown that thyroid levels do indeed normally fluctuate. In individuals with normal thyroid function the thyroid output is stimulated for example by aerobic exercise. One of the responses of the body to physical exertion is to ramp up the metabolism. Like linda187 mentioned this usually occurs within some limits considered the 'normal' range.

However, if you show signs of hyperthyroidism it has to be investigated further. Normally, around 80% of the thyroid hormone output is T4 and only 20%  is T3. If your free T4 value is normal then your thyroid might have started to produce too much T3. T4 is the precursor of T3 and T4 is converted to T3 outside the thyroid.