Download Endocrine Physiology THYROID GLAND

Endocrine Physiology
THYROID 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
Physiologic Function
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