A bit about how the thyroid gland works. The thyroid gland consists of closed follicular spaces lined by cells called cuboidal epithelium the spaces themselves being filled with a secretory substance called colloid containing the thyroid hormones. (See Fig-1).
The element iodine is required for synthesis of thyroid hormones the deficiency of which causes a disease called goiter which manifests as a swelling in the neck. Iodine is first concentrated in the cells and attached to the newly synthesized molecule thyroglobulin to form the two thyroid hormones.
Once formed the hormones bind with blood proteins and are slowly released to the tissue cells. Where they bind with intracellular proteins starting the transcription of the cellular genes resulting in the synthesis of hundreds of proteins by the cells synthetic machinery.
I should remind you that the cellular enzymes which are responsible for making metabolism possible are themselves protein in nature. So in effect the cell’s metabolism is speeded up.
That is why we see that thyroid hormone values in growing children is higher than adults as a growing body needs more energy thus a higher rate of metabolism per unit mass.
In children thyroid hormone levels have a great effect on growth. Those who are hypothyroid exhibit stunted growth. The fetal thyroid also secretes thyroid hormone and its deficiency shows up as mental retardation in later life. Such a disease is known as cretinism (See Fig-2).
As such the levels of thyroid hormones are strictly controlled by another hormone secreted from another gland the ‘thyroid stimulating hormone’ or TSH secreted from the anterior pituitary gland. A higher level of thyroid hormone causes the level of TSH to fall thus reducing thyroid hormone synthesis and vice versa.
This phenomenon is known as the ‘negative feedback control mechanism’. This TSH is again controlled by another hormone released from a part of the brain known as hypothalamus.
This hormone is called throtropin releasing hormone or TRH. This is released with the same negative feedback logic unique to human physiological systems.
It has been suggested that TRH acts as a mother controller of the thyroid system. Hormones in the blood are present in very small amounts like a billionth of a milligram per milliliter and can only be measured by a special chemical process developed for that purpose called radioimmunoassay.
Deficiency of thyroid hormone is called myxedema (See Fig-3) or hypothyroidism and can be caused by a number of factors like deficiency of iodine in the diet or decrease in synthesis.
The gland in the latter case is impaired or destroyed by the body’s own immune system antibodies a phenomenon called autoimmune disease.
Thyroid hormone is given orally as treatment while in the former case elemental iodine is given. In areas where iodine is deficient in diet salt is fortified with iodine and is called iodized salt. Deficiency of iodine produces a swelling of the gland called goiter.
Enlarged thyroid gland similar to these is also found in a disease called idiopathic non toxic colloid goiter. Hypothyroidism manifests itself as extreme lassitude, weight gain and intolerance to cold.
On the other hand hyperthyroid persons (having excess thyroid hormones) are thin, have trembling hands and fingers and have protruded eyeballs a condition called exopthalmos (See Fig-4). The stimulation to the gland comes from antibodies produced by the individuals’ immune system called thyroid stimulating immunoglobulin.
Sometimes hyperthyroidism may develop from a tumor called an adenoma. Hyperthyroidism is tackled by either giving antithyroid drugs or partially destroying the thyroid gland by giving radioactive iodine which destroys the gland by radiation or removing part of the gland surgically by an operation called partial throidectomy.
Thyroid cancer is another disease of the thyroid gland and usually manifests as a nodule which can be felt and shows an increased uptake during a diagnostic process called a thyroid scan. Like other cancers it is treated by surgery, by radiation or by chemicals called chemotherapy.
The thyroid gland also secretes another hormone called calcitonin which is produced by the Para follicular cells or C cells. This has an effect on the plasma ionic calcium concentration.
So dear reader you can realize now the complexity of the thyroid gland, nature’s metabolism regulator having a myriad variety of physiological functions but weighing only 20 grams.