Thyroid Function Tests

Question 1

What is the use of TSH?

Answer 1

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.

Question 2

How do TSH and T3 and T4 look in respects to Hyperthyroidism, Primary Hypothyroidism and Secondary Hypothyroidism?

Answer 2

Thyroid Status TSH T3 and T4

Hyperthyroidism Low High
Primary Hypothyroidism High Low
Secondary Hypothyroidism Low Low

Question 3

What antibodies can be present in thyroid disease?

Answer 3

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 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.

Question 4

What imaging techniques can be used in thyroid disease?

Answer 4

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.

Question 5

What is a radioisotope scan?

Answer 5

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