Introduction to Thyroid Markers
Nov 22, 2019
Thyroid disease is a medical condition that affects the
function of the thyroid gland (the endocrine organ found at the front of the
neck that produces thyroid hormones). The symptoms of thyroid disease vary
depending on the type. There are four general types: 1) hypothyroidism (low
function) caused by not having enough thyroid hormones; 2) hyperthyroidism
(high function) caused by having too much thyroid hormones; 3) structural
abnormalities, most commonly an enlargement of the thyroid gland; and 4) tumours
which can be benign or cancerous. Common hypothyroid symptoms include fatigue,
low energy, weight gain, inability to tolerate the cold, slow heart rate, dry
skin and constipation. Common hyperthyroid symptoms include irritability,
weight loss, fast heartbeat, heat intolerance, diarrhoea, and enlargement of
the thyroid. In both hypothyroidism and hyperthyroidism, there may be swelling
of a part of the neck, which is also known as goiter.
Both Thyroglobulin and T4 are produced by the thyroid gland,
while TSH, produced by the pituitary gland activated their production. All
three can be measured to monitor thyroid glandular function.
Thyroglobulin is the
predominant protein produced by the thyroid gland. This glycoprotein is the
precursor of thyroid hormones tri-iodothyroxine (T3) and thyroxine (T4). In
addition, it serves as a storage protein for iodide that ensures sufficiency of
this essential element. Thus, the Thyroglobulin levels may serve as a marker
for Iodine deficiency. Thyroglobulin is also produced by thyroid cancer, and
its levels are commonly used to monitor the treatment of cancer patients who
have undergone thyroidectomy. Because Thyroglobulin is also produced by normal
thyroid gland, it is not suitable as a diagnostic cancer biomarker. Blood
thyroglobulin levels can also be elevated in some cases of Grave’s disease.
The thyroid gland
produces the tyrosine-based hormone Thyroxine, also called T4 for its four
iodine moieties from its precursor Thyroglobulin. T4 levels are in high
abundance in the blood, but only after a de-ionidation step (to become T3) its
potency becomes 3-4 times higher as a hormone. This conversion can happen either
in the thyroid gland or in the target cells. The levels of T4 are 14-20 times
higher than T3 levels, thus the metabolism of carbohydrates, lipids and
proteins is regulated by the fine-tuning of T3 levels.
Both T3 and T4 act
together with its precursor Thyroglobulin as Iodine reservoir in the Thyroid,
and Iodine deficiency leads to increased T4 levels and increased size of the
thyroid gland. In the foetus and during childhood, thyroid hormones are
critical for functions such as brain development, neuronal differentiation, and
formation of neural processes.
Thyroid-Stimulating Hormone (TSH)
Thyroid-Stimulating Hormone (a.k.a Thyrotropin) is a
glycoprotein hormone secreted by the anterior pituitary gland. It activates the
thyroid gland to secrete T4 (to convert to T3 in the liver), thus being a key
regulator of normal development and metabolism.
The levels of T3 and T4 inversely feedback to the levels of TSH. Thus,
serum TSH levels are measured to diagnose and manage thyroid disorders. It is
important for clinical applications to recognize that TSH levels are subject to
circadian and ultradian rhythms.