Blood tests to evaluate the thyroid function

Blood tests are done to measure T3, T4, and TSH levels.

Thyroid stimulating hormone levels: The initial best way to test the thyroid function is to measure the level of TSH. A high TSH level indicates the thyroid gland failure and indicates primary hypothyroidism. The TSH level is low, indicative of overactive thyroid or hyperthyroidism. Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (secondary hypothyroidism).

T4 levels: T4 or thyroxine occurs in blood in 2 forms; T4 bound to proteins and free T4

The free T4 levels are more important to determine thyroid function. High levels of free T4 is indicative of hyperthyroidism and low levels of free T4 is indicative of hypothyroidism.

T3 or triiodothyronine: The elevated levels of T3 are indicative of hyperthyroidism. T3 level is rarely effective in detection of hypothyroidism since it is the last test to become abnormal.

Thyroid antibody tests

Measuring levels of thyroid antibodies help in diagnosing the cause of the thyroid problems. For example, positive anti-thyroid peroxidase and/or anti-thyroglobulin antibodies in a patient with hypothyroidism are diagnostic of Hashimoto’s thyroiditis. If the antibodies are positive in a hyperthyroid patient, the most likely diagnosis is autoimmune thyroid disease.

Non blood tests

Radioactive iodine uptake or thyroid scan: In this test a small amount of radioactive iodine is given by mouth. The amount of the radioactivity taken by the thyroid gland is determined to know whether the gland is normally functioning. Very high radioactive iodine uptake indicates hyperthyroidism and low radioactive iodine uptake indicates hypothyroidism.

Thyroid scan is done to see if the thyroid gland is functioning normally or not, to see if there is any inflammation of the thyroid gland (thyroiditis), to detect if there is any over activity of the gland. All thyroid cancers are non-functioning or cold nodules.

FNAC (Fine needle aspiration cytology)

In this a fine needle is passed through the thyroid gland and the specimen is collected for histopathological examaintion of the thyroid tissue.