The major thyroid hormone made by the thyroid gland is thyroxine, called T4. The thyroid makes a smaller amount of triiodothyronine (T3), but most of the body’s T3 comes from conversion of T4 to T3 outside of the thyroid. The production of thyroid hormones is controlled by thyroid stimulating hormone (TSH), which is made in the pituitary gland near the brain. The pituitary is always measuring the amount of thyroid hormone and adjusts the amount of TSH in order to “talk” to the thyroid. If there is less T4, TSH will go up to tell the thyroid gland to produce more, and vice versa. By listening to each other, TSH and T4 keep each other balanced in a fairly narrow range.

TSH: Testing TSH is the best way to check thyroid function initially. A high TSH level indicates that the thyroid hormone level is low (usually hypothyroidism). A low TSH usually means that there is too much thyroid hormone. This happens when the thyroid is overactive (hyperthyroidism), but the most common reason is when someone is taking too much thyroid medication.

• T4: Tests of T4 are measuring the thyroid hormone itself. The most usual test is free T4 (FT4) because this measures the T4 hormone that is free to be used. FT4 is most useful to see the severity of a thyroid problem when TSH is abnormal. More rarely, an abnormal FT4 is necessary to diagnose problems of the pituitary gland.

• T3: T3 tests, either total T3 or free T3, can be useful to evaluate hyperthyroidism. However, T3 testing rarely is helpful in patients with hypothyroidism because TSH and FT4 will be abnormal earlier than T3. Reverse T3 is another form of thyroid hormone that is inactive, and measurement does not help determine whether hypothyroidism exists or not.

Yes! There are many medications that may affect thyroid status. One important example is Biotin, which is a commonly taken over-the-counter supplement, can cause thyroid function tests appear to look abnormal when they are actually normal in the blood. Biotin should not be taken for 2 days before blood is drawn for thyroid function testing to avoid this effect.