Endocrine PhysiologyTHYROID GLAND

Dr. Meg-angela Christi Amores

Where is your thyroid gland located?

• located immediately below the larynx on each side of and anterior to the trachea

• normally weighing 15 to 20 grams in adults

What are the hormones of the thyroid gland?

• two major hormones:– Thyroxine(T4) and triiodothyronine(T3)– increase the metabolic rate of the body– Thyroid secretion is controlled primarily by

thyroid-stimulating hormone (TSH) secreted by the anterior pituitary gland

• also secretes calcitonin

Synthesis and Secretion

• 93% of metabolically active hormones secreted by the thyroid gland = THYROXINE (T4)

• 7% = TRIIODOTHYRONINE (T3)• Triiodothyronine is about four times as potent as

thyroxine

Synthesis and Secretion• Physiologic Anatomy– Composed of large

numbers of closed follicles

– filled with a secretory substance called colloid and lined with cuboidal epithelial cells that secrete into the interior of the follicles

Synthesis and Secretion

• Thyroglobulin – major constituent of the colloid– contains the thyroid hormones within its molecule

• Iodine– Required for formation of thyroxine– 50 milligrams of ingested iodine in the form of

iodides are required each year, or about 1 mg/week

• Transport of iodides into follicles (iodide pumping)• conversion to an oxidized form of iodine• Each molecule of thyroglogulin contains 70 tyrosine

amino acids• Oxidized Iodine (I3)is capable of combining directly

with the amino acid tyrosine• Oxidation is promoted by enzyme PEROXIDASE and

accompanied with HYDROGEN PEROXIDE

• binding of iodine with the thyroglobulin molecule is called organification of the thyroglobulin

• successive stages of iodination of tyrosine and final formation of the two important thyroid hormones, thyroxine and triiodothyronine

• Tyrosine is first iodized to monoiodotyrosine

• and then to diiodotyrosine

• Then, during the next few minutes, hours, and even days, more and more of the iodotyrosine residues become coupled with one another

Transport of thyroid hormones

• Thyroxine and Triiodothyronine Are Bound to Plasma Proteins– 99% of thyroid hormones are bound to:• thyroxine-binding globulin• thyroxine-binding prealbumin and albumin

• Thyroid Hormones Have Slow Onset and Long Duration of Action

• T4: long latency >2-3 days, maximumum in 10 – 12 days• T3: latent: 6 to 12 hours , maximum in 2 – 3 days

Phys

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Func

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Physiologic Function

• Thyroid Hormones Increase the Transcription of Large Numbers of Genes

• Thyroid Hormones Increase Cellular Metabolic Activity

• Effect on growth: • thyroid hormone is essential for the metamorphic

change of the tadpole into the frog• promote growth and development of the brain during

fetal life and for the first few years of postnatal life

Physiologic Function

• Effects on specific bodily mechanisms– Stimulation of Carbohydrate Metabolism– Stimulation of Fat Metabolism– Increased Requirement for Vitamins– Increased Basal Metabolic Rate– Decreased Body Weight

Regulation of Thyroid hormone