TESTS FOR DISEASES OF THYROID GLAND
Thyroid gland produces chemicals which are known as thyroid hormones, Thyroxine (T4) and Triiodothyronine (T3). T3 is also produced by the peripheral tissues by removal of an iodine atom. Thyroid gland growth and its functions are controlled by the hormone Thyroid Stimulating Hormone (TSH) which is secreted by pituitary gland located under the brain. Thyroid and Pituitary gland work in a system of feedback, whereby more TSH is secreted in response to low T4; thus stimulating the thyroid gland to produce more of T4.
The various investigations performed to detect diseases of thyroid are based on tests which assess thyroid structure and its functions.
Tests for Thyroid Function:
Blood tests to measure T4 and T3 (Free and Total), TSH are used to assess thyroid function.
The blood should be taken in fasting state. Patients should not take Thyroxine tablets on the day of test, before withdrawal of blood sample, if they are on Thyroxine. However, after giving blood sample, they can take Thyroxine as usual.
A low level of T4 (Hypothyroidism) may be due to inability of thyroid gland to produce adequate T4 (Primary Hypothyroidism) where there is a high TSH level (response of Pituitary to low T4). In case of low T4 due to Pituitary producing inadequate TSH (Secondary or Central hypothyroidism), TSH is low or normal with a low T4.
In cases with high T4 (Thyrotoxicosis), TSH levels are low due to a negative feedback on the Pituitary. The reasons may be as follows-
- This may be due to inflammation of Thyroid gland (Thyroiditis), where preformed stored hormones are released into circulation from Thyroid gland itself; it is not the result of functioning in excess of normal.
- This biochemical abnormality may also occur when the thyroid gland is hyperfunctioning (Hyperthyroidism). It can be differentiated from the above said Thyroiditis by a Radioisotope Thyroid Scan (Nuclear Scan).
Levels of TSH are extremely sensitive to the levels of T4 and hence in most situations, reflect thyroid functional status. High TSH levels suggest a reduced thyroid function (Hypothyroidism) and Low TSH is seen in increased thyroid function (Hyperthyroidism).
T4 circulates in the blood in two forms: protein-bound (bound T4) and Free T4 (FT4), which can enter the various target tissues to exert its effects. The free T4 fraction is the most important to determine how the thyroid is functioning. Individuals who have hyperthyroidism will have an elevated FT4 whereas patients with hypothyroidism will have a low level of FT4. Performing TSH with the FT4 accurately determines the thyroid function.
In some situations of hyperthyroidism, T3 tests are often useful to diagnose or to determine the severity of the hyperthyroidism. In some individuals with a low TSH, only the T3 is elevated and the FT4 is normal (T3 Toxicosis).
In all other conditions, T3 testing is not of much utility.
While in most situations, doing Total T4 (Free + Bound) would suffice, in certain situations like in pregnancy or in women on drugs which increase T4-binding protein, Total T4 may be high however a Free T4 would be the true test of thyroid function.
Some disorders of thyroid gland function are due to antibodies formed against the thyroid gland by our own immune system.
Anti-TSH receptor antibodies stimulate the thyroid gland as done by TSH hormone to produce excess thyroid hormone (Hyperthyroidism) in a disease called Graves’ disease. These antibodies also act on eyes and skin to produce various effects of the disease on these organs.
Another set of thyroid antibodies, which are important in causing hypothyroidism are Anti Thyroid Peroxidase antibody (anti TPO) and Anti Thyroglobulin antibody (anti TG). Positive anti TPO and/or anti TG antibodies in a patient with hypothyroidism makes a diagnosis of Hashimoto’s thyroiditis.
Anti-TPO antibodies are performed to detect the cause of Hypothyroidism or Thyroiditis (Thyroid gland inflammation). However, this test need not be repeated to assess response to treatment as the treatment of hypothyroidism or hyperthyroidism is directed at thyroid function alone and not at these antibodies.
Thyroglobulin (Tg) is a protein produced by normal thyroid cells and also thyroid cancer cells. It is used most often in patients who have had total surgical removal of thyroid gland for cancer to monitor them after treatment for recurrence of tumor.
THYROID SCAN (RADIOACTIVE IODINE UPTAKE / PERTECHNATE SCAN)
Thyroid gland is extremely efficient in picking up Iodine from circulation for the synthesis of thyroid hormones. This becomes even more effective in conditions where the gland is functioning in excess. Therefore, the activity of the gland can be measured by use of giving radioactive iodine to a patient and measuring its activity in the thyroid gland. By measuring the amount of radioactivity that is taken up by the thyroid gland (radioactive iodine uptake, RAIU), doctors may determine whether the gland is functioning normally or in excess/reduced.
In addition to the radioactive iodine uptake, a thyroid scan by another tracer called Technetium pertechntate may be obtained, which shows a picture of the thyroid gland as per its activity. A very high RAIU is seen in individuals whose thyroid gland is overactive (hyperthyroidism), while a low RAIU is seen when the thyroid gland is underactive (hypothyroidism or Thyroiditis).
The images of the pattern of distribution of uptake by the thyroid gland can show whether it is diffuse (generalized all over the gland) or patchy (single nodule or a multinodular gland).
STRUCTURE OF THYROID GLAND:
Ultrasound (USG) scan of the thyroid is the commonest means of assessing the structure of thyroid gland. There may be a generalized increased in size (diffuse goiter) or nodules (single or multiple). USG combined with Doppler studies can also give details of shape, consistency and internal blood supply within the thyroid gland and its nodules which can help in suspecting cancer or non cancer nature of these nodules.
USG also helps in directing the pathologist in performing a needle test known as fine needle aspiration cytology (FNAC) of these nodules for diagnosis of cancers of thyroid.
CT scan or MRI scans also may be done in certain situations where USG is not adequate for showing a massively enlarged thyroid gland.
HOW WILL YOUR DOCTOR APPROACH YOUR THYROID DISEASE
After the necessary history taking and clinical examination including examination of the thyroid gland itself, the doctor will order thyroid function test which includes TSH, Total or Free T4 and T3 (if required). Based on these tests, hypothyroidism, hyperthyroidism or normal thyroid function status will be established. Antibodies like Anti-TPO antibodies or TSH Receptor antibodies may be required in case of suspected autoimmune thyroid disease (Hashimoto’s Thyroiditis) or Graves’ disease.
In case of mildly increased TSH and normal T4, the doctor may order the anti TPO antibodies to decide further course of action.
In case of suspected hyperfunctioning thyroid gland, Iodine or Pertechnate scan may be ordered for confirmation by the physician.
In case of enlarged thyroid gland (Goiter), ultrasound of the thyroid gland will be done and FNAC if needed, to assess the cellular structure of the thyroid swelling.
In hypothyroidism, blood tests of thyroid functions are repeated once in 2-3 months initially and later 6-12monthly to monitor the response to treatment. In cases of hyperthyroidism or in pregnant women with thyroid disease, this monitoring may be asked for more frequently.
Enlarged thyroid gland, nodules of thyroid which are not of cancerous nature may be followed up periodically using USG and other tests.
This page is edited by Col (Dr) J Muthukrishnan, SM.
Dr. Muthukrishnan is an expert Endocrinologist and is presently working as an Associate Professor at Armed Forces Medical College, Pune (Maharashtra) India.