Thyroid Gland: Structure and Hormones

Thyroid Gland Structure

• A butterfly-shaped gland weighing 20–40 g.
• Located immediately inferior to the thyroid cartilage of the larynx and anterior to the trachea.
• Composed of left and right lobes connected at the anterior midline by a narrow isthmus.

Histological Level

• Spherical structures called thyroid follicles compose the thyroid gland.
• Simple cuboidal epithelial cells, known as follicular cells, form the follicle wall.
• Follicular cells secrete thyroid hormones and surround a central lumen.
• The central lumen contains protein-rich fluid termed colloid.
• Follicular cells become columnar during high activity and flat when inactive.
• C (clear) cells, also named parafollicular cells, are large, pale cells that lie in between the follicles.
• C (clear) cells (parafollicular cells) produce the hormone calcitonin (CT).

Thyroid Hormones

• Thyroxin (tetraiodothyronin or T4) is a thyroid hormone.
• Triiodothyronin (T3)is a thyroid hormone.
• Calcitonin, a calcium-lowering hormone, is a thyroid hormone.

Synthesis of Thyroid Hormones

• Thyroglobulin (TG) is synthesized in the endoplasmic reticulum (ER) of thyroid follicular cells.
• TG is then transported into the follicular lumen.
• Two iodide molecules are transported into the follicular cell by the sodium-iodide symporter (NIS).
• Iodide diffuses to the apical surface and is transported into the follicular lumen by the pendrin (P) carrier.
• Thyroid peroxidase(TPO) oxidizes iodide.
• The oxidized iodide is linked to tyrosine residues in TG to form diiodotyrosine (DIT) and monoiodotyrosine (MIT) molecules.
• Within TG, T4 is formed from two DIT molecules.
• T3 is formed from one DIT and one MIT molecule.
• TG containing T4 and T3 is resorbed into the follicular cell via endocytosis.
• Lysosomal enzymes degrade TG to release T4 and T3 molecules.
• T4 and T3 molecules then move across the basolateral membrane of the follicular cell into the adjacent capillaries.
• Iodine is required by the thyroid gland to produce thyroid hormones.
• All steps in thyroid hormone production are stimulated by TSH.

Control of Thyroid Function

• Secretion of thyroid hormones is regulated by: Thyrotropin-releasing hormone (TRH)
• TRH is secreted from the median eminence of the hypothalamus (neurosecretion).
• TRH reaches the anterior pituitary through the hypothalamo-hypophyseal portal circulation to stimulate TSH secretion.
• Secretion of thyroid hormones is regulated by: Thyrotropin (thyroid stimulating hormone = TSH)
• TSH is secreted by the basophil cells of the anterior pituitary.
• TSH increases the size, vascularity, and function of the thyroid gland.
• Secretion of thyroid hormones is regulated by: Thyroid hormones level in the blood (feedback control)
• T4 & T3 in plasma inhibit TSH secretion by direct inhibition of the anterior pituitary and the hypothalamus.

Functions of Thyroid Hormones

• Thyroid hormones increase the rates of the chemical reactions of many cells in the body (the metabolic rate).
• Thyroid hormones increase O2 consumption and heat production and increase body temperature.
• Normal physiological levels of thyroid hormones stimulate protein synthesis and growth (anabolic effect).
• Excess thyroid hormones cause protein catabolism.
• Intestinal absorption of glucose increases due to thyroid hormones.
• Liver glycogenolysis increases due to thyroid hormones.
• Gluconeogenesis increases due to thyroid hormones.
• Thyroid hormones increase lipolysis (free fatty acids in plasma) and decrease cholesterol in the blood.
• Thyroid hormones stimulate linear growth of bone.
• Thyroid hormones stimulate the eruption and development of teeth.
• Thyroid hormones stimulate normal muscular development and function.
• Thyroid hormones stimulate growth of skin and hair follicles.
• Thyroid hormones are essential for CNS development, growth, and function during fetal life and in the first few years after birth.
• Thyroid hormones are essential for the myelination of nerve fibers.
• Thyroid hormones are essential for the development of synapses.
• Thyroid hormones stimulate gonadal functions.
• They are essential for the normal menstrual cycle and spermatogenesis & fertility.
• Thyroid hormones stimulate milk secretion during lactation.
• Thyroid hormones increase heart rate and strength of cardiac contraction.
• Thyroid hormones increase cardiac output due to increased stroke volume and heart rate.
• Thyroid hormones increase the number of alpha-adrenergic receptors in the heart.
• Respiration is increased due to increased body metabolism.
• Appetite is increased due to thyroid hormones.
• Intestinal motility is increased due to thyroid hormones.
• Thyroid hormones enhance wakefulness, alertness, & responsiveness to various stimuli.
• Thyroid hormones promote normal hydration and secretory activity of the skin.

Iodine Deficiency

• Iodine deficiency in diet can cause: -An enlarged thyroid (goiter).

Sexual Growth Retardation

• Sex organs remain infantile in sexual growth retardation.

Metabolism

• Basal metabolic rate (BMR) is decreased in slower metabolism.
• Those affected are sensitive to cold (cold intolerance).
• Serum cholesterol is increased in slower metabolism.

General Features

• Swollen eyelids with narrow palpebral fissures are present.
• Depressed nose with wide nostrils is present.
• Enlarged protruded tongue between thick lips are present.
• Bulging is due to muscle weakness, together with constipation.
• Skin is cold, pale, dry & coarse with scanty hair.

Cretinism vs Dwarfism

Myxoedema (Hypothyroidism in Adults)

• Myx is mucin + edema is swelling.

Causes of Hypothyroidism

• Primary hypothyroidism is due to causes within the thyroid.
• Hashimoto's Thyroiditis Is an autoimmune destruction of the thyroid with lymphocytic infiltration.
• Secondary Hypothyroidism occurs.
• Hypothalamic Hypothyroidism is dueto a failureof hypothalamus to secrete TRH.
• Pituitary Hypothyroidism is due to failure of pituitary to secrete TSH.

Clinical Features of Hypothyroidism

• Low intolerance.
• Puff leths.
• Non(Prote ar spaces water ar S.
• Dec men.
• Rou.
• Constipation, decreased appetite.

Hyperthyroidism

• Graves' disease is toxic goiter and thyrotoxicosis.

Causes of Hyperthyroidism

• Grave's disease: (1y hyperthyroidism).
• It is an autoimmune disease in which antibodies [Thyroid-stimulating immunoglobulin (TSI)] bind with TSH receptors result in the continuous stimulation of thyroid hormone production and secretion as well as thyroid growth (goiter).
• TSH-secreting tumor of the pituitary: (2ry hyperthyroidism).
• It is very rare.

Clinical Features of Hypothyroidism

• Increased basal metabolic rate (BMR).
• Intolerance to heat.
• Nervousness, restlessness, anxiety, insomnia.
• Hyperglycemia and decreased cholesterol.
• Weight loss despite increased food intake.
• Increased resting heart rate.
• Fine tremors in hands.
• Diarrhea.
• Excessive sweating, warm moist skin.
• Eye: lid retraction producing a staring look, and lid lag and exophthalmos (protrusion of the eyeball.)

Primary Hypothyroidism

• Low T4
• Low TRH
• High TSH

Secondary Hyperthyroidism

• High T3 and T4 is present
• Low TRA
• Hig Tsh

Goiter

• A goiter can develop in all of the disorders except:

  Disorder	T4	TSH	TRH
  Primary Hypothyroidism	↓	↑	↑
  Pituitary Hypothyroidism	↓	↓	↑
  Pituitary Hyperthyroidism	↑	↑	↓
  Graves' Disease (Autoimmune)	↑	↓	↓