Is a tsh of 5.280 (range: 0.45-4.500) serious or can it be ignored?

Needs Monitor: This TSH is not significantly high and some Endocrine physicians would not even consider it abnormal unless it was greater than 6. I would say that you will need monitoring and repeat testing but at this time it is not really anything to worry about.

Possible hypothyroid: An elevated TSH typically means that your thyroid level may be low. With a TSH of 5.28, the thyroid level (free T4) is unlikely to be significantly low, and will probably still be in the normal range. In your age group, the most likely cause is autoimmune thyroiditis. Checking thyroid antibodies (TPO) can help make the diagnosis. Unless you have low thyroid symptoms, you may not need Rx yet.

Should not ignore: All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Having said that, You may not have any symptoms due to being in early stages of thyroid deficiency. If you wait for symptoms to develop, it may have cause harm by then. It would be prudent to see your doctor about replacement thyroid medication.