An increase in serum free thyroid hormone levels generally means two things: the first is what is often referred to as primary hyperthyroidism, which is a diffuse hyperplasia and hypertrophy of thyroid cells due to an immune disorder, resulting in a significant increase in the secretion of thyroid hormones, which can lead to a range of clinical hypermetabolic symptoms. The second condition means that the thyroid cells have been damaged, such as in subacute thyroiditis or Hashimoto’s thyroiditis, which can cause damage to the thyroid follicular cell membranes for various reasons. This increase in hormone levels is transient, and as the condition gradually improves hormone levels can gradually decrease to within normal limits.