Normal T4 values in a hypothyroid patient may occur in the following conditions:

1. In early or very mild hypothyroidism. Serum TSH values are usually elevated. Some investigators have reported patients in whom the T4 and TSH values were both within reference range, but the TRH test result exhibited an exaggerated response suggestive of hypothyroidism.
2. When the T4 level in a hypothyroid person is artifactually increased into the normal population reference range. This may be due to elevated TBG levels or because the reference range overlaps with values for some patients with mild hypothyroidism. The TSH level should be elevated in most cases, unless the reason for the patient’s hypothyroidism is pituitary insufficiency.