Introduction

The thyroid gland is a butterfly-shaped endocrine gland that is normally located in the lower front of the neck.

The main thyroid hormone secreted by the thyroid gland is thyroxine (T4) as it comprises of four iodine atoms. Triiodothyronine (T3) is formed from T4 by removing an iodine atom. Thyroid stimulating hormone (TSH) is secreted by pituitary gland which controls the secretion of T4 from thyroid gland.

T4 and T3 is bound to specific transport proteins when in circulation. If the concentration of these transport proteins varies, there can be variation in bound T4 and T3 concentration. The “free” T4 or T3 is the unbound hormone which is capable of entering and affecting the body tissues.

Blood tests for Thyroid Functions

TSH

Variation in TSH is as an early sign frequently happening before the concentration of thyroid hormones in the body actually decreases or increases. An increases TSH concentration specifies that the thyroid gland is not synthesizing adequate thyroid hormone (primary hypothyroidism). If concentration of TSH is low, it specifies that the thyroid gland is generating more thyroid hormone (hyperthyroidism). Sometimes, TSH level reduces because of abnormality in the pituitary gland that stops it from synthesizing TSH for stimulating the thyroid (secondary hypothyroidism). Normally, a normal TSH concentration shows that the thyroid is in normal condition.

T4

Total T4 is the measurement of both the bound and free hormone and can vary when binding proteins varies. Free T4 level measures the T4 that enter and affect the body tissue. Free T4 (FT4) measures the functioning of thyroid gland. An increased TSH and reduced FT4 shows primary hypothyroidism occurs due to damage in the thyroid gland. A reduced TSH and reduced FT4 specifies hypothyroidism because of defect in the pituitary gland. A reduced TSH along with an increased FT4  indicates hyperthyroidism.

T3

T3 level is significant in diagnosing hyperthyroidism or for determining the progression of the hyperthyroidism. In hyperthyroidism, T3 level is increased. In some hyperthyroidism, reduced TSH, only the T3 is increases and the FT4 is normal. T3 level is less significant in the hypothyroidism as it is the last test which becomes abnormal. In some severe hypothyroidism, increased TSH and reduced FT4, but normal T3 is observed.

Free T3

Measurement of free T3 is generally not reliable.

Thyroid Antibody Tests

In many cases of hypothyroidism or hyperthyroidism, lymphocytes generate antibodies against thyroid cell proteins (thyroid autoimmunity). Two example of such antibodies are thyroglobulin antibody and thyroid peroxidase antibody. Level of thyroid antibodies indicate the cause of the thyroid disorder. 

Thyroglobulin

Thyroglobulin (Tg) is not generally used to measure thyroid function but it is done for monitoring treatment of post-surgery in thyroid cancer.