Thyroid Function Tests

Thyroid Stimulating Hormone (TSH)

If you are concerned about possible thyroid disease you can use TSH alone as a screening test. When TSH is abnormal, then you can measure T3 and T4 to find out more information.

In hyperthyroidism, TSH is suppressed by the high thyroid hormones so you get a low TSH level. The exception is a pituitary adenoma that secretes TSH in which case it is high.

In hypothyroidism, TSH is high as it is trying to stimulate more thyroid hormone release. The exception is a pituitary or hypothalamic cause of the hypothyroid (secondary hypothyroidism), in which case the TSH level will be low.

 

T3 and T4

In hyperthyroidism you expect raised T3 and T3 and suppressed TSH. In hypothyroidism you expect low T3 and T4 and raised TSH.

Thyroid Status

TSH

T3 and T4

Hyperthyroidism

Low

High

Primary Hypothyroidism

High

Low

Secondary Hypothyroidism

Low

Low

 

Antibodies

Antithyroid Peroxidase (anti-TPO) Antibodies are antibodies against the thyroid gland itself. They are the most relevant thyroid autoantibody in autoimmune thyroid disease. They are usually present in Grave’s Disease and Hashimoto’s Thyroiditis.

Antithyroglobulin Antibodies are antibodies against thyroglobulin, a protein produced and extensively present in the thyroid gland. Measuring them is of limited use as they can be present in normal individuals. They are are usually present in Grave’s Disease, Hashimoto’s Thyroiditis and thyroid cancer.

TSH Receptor Antibodies are autoantibodies that mimic TSH, bind to the TSH receptor and stimulate thyroid hormone release. They are the cause of Grave’s Disease and so will be present in this condition.

 

Imaging

Thyroid Ultrasound

Ultrasound of the thyroid gland is useful in diagnosing thyroid nodules and distinguishing between cystic (fluid filled) and solid nodules. Ultrasound can also be used to guide biopsy of a thyroid lesion.

 

Radioisotope scan

Radioisotope scans are used to investigate hyperthyroidism and thyroid cancers. Radioactive iodine is given orally or intravenously and travels to the thyroid where it is taken up by the cells. Iodine is normally used by thyroid cells to produce thyroid hormones. The more active the thyroid cells, the faster the radioactive iodine is taken up. A gamma camera is used to detect gamma rays emitted from the radioactive iodine. The more gamma rays that are emitted from an area the more radioactive iodine has been taken up. This gives really useful functional information about the thyroid gland:

  • Diffuse high uptake is found in Grave’s Disease
  • Focal high uptake is found in toxic multinodular goitre and adenomas
  • “Cold” areas (i.e. abnormally low uptake) can indicate thyroid cancer

 

Last updated November 2018
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