Is TSH testing sufficient or should doctors routinely test T4 and T3 levels?

In the majority of people, a TSH test alone is offered as a first test for assessing thyroid function. If the TSH level is abnormal, further tests (FT4 and sometimes FT3)  may then be offered to patients. Antibody tests may also be offered to confirm whether the cause is autoimmune.

In people recently treated with radioactive iodine for hyperthyroidism, children and young people, or patients with known or suspected pituitary disease, FT4 should also routinely be checked. 

In more than 99% of people with a normal TSH, and who have no other reason to have a thyroid or pituitary problem, FT4 and FT3 will also be normal.

In someone without thyroid disease, if the TSH is raised and hypothyroidism is suspected, then it should be repeated and FT4 tested. If persistently raised, then an anti-TPO Ab test may be offered. This is generally only tested on one occasion. If the TPO antibodies are positive, it is more likely the thyroid level may deteriorate over the coming months or years.  Thyroglobulin antibodies (TgAB) generally do not add much to anti-TPO Ab results.

If antibodies are negative, it means the person likely does not have a thyroid disorder and that the high TSH may resolve spontaneously, or there is an underlying thyroid disorder caused by another factor (e.g. following a viral infection or due to prescribed medication).

If the TSH is low, measuring FT3 and FT4 will diagnose hyperthyroidism. If confirmed, a TSH receptor antibody test looking for Graves ľ disease may be offered. In patients with known or suspected pituitary disease, the TSH level is misleading and FT4 should be measured. This is also true in other situations, such as within three months of treatment of hyperthyroidism (when the TSH may stay lower than it should be) and with some genetic abnormalities of thyroid function. If a patient has a normal TSH but persistent symptoms suggestive of hyperthyroidism or hypothyroidism, the measurement of FT4 and FT3 on one occasion is helpful to rule out a pituitary problem or these other rare conditions.

When monitoring patients already on thyroid hormone, TSH measurement alone is usually sufficient. If the TSH is not normal, FT4 (and possibly FT3) should also be measured. An exception is patients taking T3 alone, or in mixed preparation, when a FT3 measurement might be helpful. Follow-up antibody tests are not normally required.

NICE guidance on the assessment and management of thyroid disease