I just had a thyroid uptake scan. 13.8% what does this mean?

MAY BE NORMAL. Normal range of thyroid uptake varies from lab to lab and geographic region to region. For those labs who accepts 10% to 30% as normal range, the 13.8% may be considered normal but lower range of normal.
Normal uptake. This percentage refers to the percentage of radioiodine uptake at 24 hours after its administration. In your case, this means that 13.8% of the radioactive iodine you ingested during the test is present in your thyroid gland 24 hours later. A normal uptake usually varies between 10 and 30% at 24 hours. Other parameters such as thyroid hormone levels are used as well to verify the status of thyroid.
Possible hypothyroid. Thyroid uptake depends on thyroid iodine trapping which is proportional to the rate at which the thyroid is making hormone. The total amount of iodine in the diet is also a major factor. If you use iodized salt and live by the seashore your body will have more iodine available to dilute the test dose; thus giving a lower uptake value. High sensitivity TSH is a much more reliable indicator.