Thyroid Hormones

The thyroid is a small gland, shaped like a butterfly, located in the lower part of your neck.


The function of a gland is to secrete hormones. The main hormones released by the thyroid are triiodothyronine, abbreviated as T3, and thyroxine, abbreviated as T4. The problems with the thyroid can be:

1. Hypothyroidism:
This is an underactive thyroid so produced not sufficient amounts of T3 and T4. Symptoms of hypothyroidism usually go along with a slowdown in metabolism, and can include fatigue, weight gain, and depression, among others. The most common cause of hypothyroidism is an autoimmune disease called Hashimoto disease.

2. Hyperthyroidism:
This is a thyroid which become overactive in producing the thyroid hormones. The symptoms are goiter, fatigue, weight changes, heart beat and blood pressure problems. Also here the most common cause is an autoimmune disease called Graves diseases.

3. Thyroid Cancer

4. Thyroiditis:

Thyroiditis is an inflammation of the thyroid.

Synthesis and release of the thyroid hormones:
Thyroid hormone synthesis and release is controlled by the hypothalamic-pituitary axis involving negative feedback control. Thyrotrophin-releasing hormone (TRH) is released from neurosecretory cells in the hypothalamus and travels via portal capillaries to the anterior pituitary where it stimulates the release of TSH into the general circulation. It is bound by TSH receptors in the thyroid gland which stimulate synthesis of the iodine pump in the follicular cells and the production of thyroglobulin and thyroperoxidase, with the effect of an in an increase in the release of the thyroid hormones. Circulating levels of thyroid hormones exert negative feedback control on the TSH secretion. Conversely, when thyroid hormone levels are low there is stimulation in the secretion of TSH.

Thyroid function tests
These comprise the thyroid hormones (T4 and T3, total or free), thyroid stimulating hormone (TSH) and thyroid autoantibodies: anti-Thyroid peroxidase (TPO) and anti-thyroglobulin (TG). Thyroid function tests can demonstrate the presence or the absence (euthyroid) of thyroid dysfunction but further investigations and clinical assessment are necessary to determine the cause and select appropriate treatment. This is a cancer of the thyroid nodules or tissue.

Interpretation of Thyroid function tests

TSH will be suppressed to below the reference range and total or free T4 will be elevated. If the TSH is suppressed and the total or free T4 is normal, a total or free T3 should be measured. If this is elevated it is indicative of T3 thyrotoxicosis. If the total or freeT3 is normal, the TSH could be suppressed due to non-thyroidal illness.
TSH will be significantly elevated and total or free T4 below the reference range. Thyroxine replacement is indicated by such results. It is frequently found that there is a moderate elevation of TSH with total or freeT4 in the normal range; the pattern of sub-clinical or compensated hypothyroidism. A positive TPO antibody result indicates that autoimmune disease is present. 

Thyroid Markers

TSH: Thyroid Stimulating Hormone is a glycoprotein hormone secreted by the anterior pituitary gland. TSH is a glycoprotein and consists of two subunits, the Alfa and Beta subunit, the Alfa subunit is the same as some other glycoprotein hormones (e.g. HCG, FSH), while the beta subunit specific is for the hormone. TSH that is elevated, or above normal, is considered indicative of hypothyroidism, while a low TSH or below normal, is considered evidence of hyperthyroidism.

T4 and Free T4: L-thyroxine (3, 5, 3-, 5-L-tetraiodothyronine (T4)) is produced by the thyroid gland, circulates in the blood bound to plasma proteins like thyroxine-binding globulin (TBG), thyroxine binding prealbumin and albumin . Approximately only 0.03% of the total circulating thyroxine is unbound. This free T4 (FT4) is the physiologically active portion of the hormone which stimulates the metabolism. The measurement of total serum T4 (bound + free) to assess the clinical status of the thyroid gland is not diagnostically accurate when significant changes occur in the serum binding proteins . Alterations in TBG concentration, pregnancy, oral contraceptives, estrogen therapy or drugs which alter the binding of thyroxine to the carrier proteins may cause corresponding changes in the total T4 when unbound free thyroxine levels remain relatively unchanged. Therefore, measurement of the free T4 (FT4) typically correlates more closely to the patient's actual thyroid status than the total.

T3 and Free T3: Triiodothyronine (T3) is present in human serum in an equilibrium mixture of bound and free forms, with approximately 0.4% of the total T3 circulating as Free Triiodothyronine (FT3). Like FT4 all change in the serum concentration of binding proteins will cause a change in the concentration of total T3 while FT3 remain unchanged. Direct measurement of free T3 enables thyroid function examination even in the presence of abnormal liver function, hormone fluctuation during pregnancy and variations in levels of serum binding proteins.

Thyroid autoantibodies: Both Graves’ disease leading to hyperthyroidism and Hashimoto’s disease (autoimmune thyroiditis) which is predominantly associated with hypothyroidism, are manifestations of autoimmune disease. Graves’ disease is characterised by the presence of TSH receptor stimulating antibody. Hashimoto’s disease is usually accompanied by the presence of circulating antibodies to thyroperoxidase and thyroglobulin. Thyroperoxidase antibody (anti-TPO antibody) is more reliably associated with Hashimoto’s disease than thyroglobulin antibody. TPO antibodies can occur in other thyroid disorders including transient destructive thyroiditis, often post-partum.

TG: Thyroglobulin (Tg) is a protein produced only by the Thyroid. The thyroid is a small gland, shaped like a butterfly, located in the lower part of the neck.

The function of the gland is to secrete hormones. The main hormones released by the thyroid are tri-iodothyronine, abbreviated as T3, and thyroxin, abbreviated as T4. These hormones, which are present in a free (FT4 and FT3) and bound form, help to regulate the rate at which the body uses energy.

The measurement of the Tg in blood is an important laboratory test for checking whether a patient still has some thyroid activity present.

When a patient has had their thyroid completely removed, the measurement of Tg in a blood sample can be used to check whether any tumour cells were left behind. (Like for PSA)