THE THYROID GLAND
Anatomical Structure
Gross Anatomy
• Located in neck
– lobes
– isthmus
• Relations
– Larynx
– Trachea
– Recurrent laryngeal
nerves
– Parathyroid glands
– Carotid sheath
• Blood supply
– Sup. thyroidal a.
– Inf. thyroidal a.
Embryology
• Thyroglossal duct
• Foramen caecum
• Thyroid cysts
Histology
• Thyroid follicles
– Simple cuboidalcolumnar
• Colloid
– Thyroglobulin
• Rich vascularization
• Parafollicular cells
Thyroid Hormone
• Tyrosine
• 3-Monoiodotyrosine
(MIT)
• 3,5-Diiodotyrosine (DIT)
• 3,5,3’,5’-Tetraiodothyronine
(T4)
• 3,5,3’-Triiodothyronine
(T3)
• 3,3’,5’-Triiodothyronine
(Reverse T3, rT3)
Thyroid Hormone
Synthesis RC
C=O
N
OH-
CH3CH
C=O
RC
• Tyrosine in Thyroglobulin
Thyroid Hormone
Synthesis RC
I
C=O
N
OH-
CH3CH
C=O
RC
• Thyroperoxidase attaches Iodine to 3
position---MIT
Thyroid Hormone
Synthesis RC
C=O
I
N
CH3CH
OH-
C=O
I
RC
• Thyroperoxidase attaches Iodine to 5
position---DIT
Thyroid Hormone
Synthesis RC
I
C=O
I
N
OH-
CH3CH
O
C=O
I
I
RC
• Thyroperoxidase attaches ring from one
DIT to adjacent DIT = Thyroxine (T4)
Thyroid Hormone
Synthesis RC
I
C=O
I
N
OH-
CH3CH
O
C=O
I
RC
• Thyroperoxidase attaches ring from one
MIT to adjacent DIT = Triiodothyronine (T3)
Thyroid Hormone
Synthesis RC
I
I
C=O
N
OH-
O
CH3CH
C=O
I
RC
• Thyroperoxidase attaches ring from one
DIT to adjacent MIT = Reverse T3
Thyroid Hormone
Biosynthesis
•
•
•
•
•
Iodine pump/trap
Thyroglobulin
Iodination
Thyronine
Thyroglobulin
pinocytosis
• Hormone release
Thyroid hormone
transport
• Thyroid binding globulin (TBG)
– Highest affinity
• Thyroid binding prealbumin
– Intermediate affinity
• Albumin
– Low affinity, High capacity
Thyroid Hormone
• T3 is 3-8X more active than T4
• Thyroid Gland produces 10X T4 to T3
• 5’-deiodinase
– Converts T4 to T3 in Target Tissues
– Primarily responsible for circulating
levels of T3
• T4 probably a pro-hormone
• Thyroid hormone receptor
– Nuclear
Physiological effects
of thyroid hormone
• Increases oxygen consumption and
heat production
• Positive chronotropic and inotropic
effects on heart
• Increase sensitivity to adrenergic
effectors
– Up-regulates -adrenergic receptors
• Increase gut motility
• Increase bone turnover
Physiological effects
of thyroid hormone
• Increases reflex response
• Increase hepatic glycogenolysis and
gluconeogenesis
• Stimulates lipolysis
• Developmental effects
– Growth
– Brain development
HIGHER BRAIN OF THE
REGULATION
CENTERS
THYROID
GLAND
BODY
SHORT-LOOP
TEMP.
FEEDBACK
HYPOTHALAMUS
EXPOSURE
TO COLD TSH
TSHRECEPTOR
ANTIBODIES
THYROID AUTO
REGULATION
TRH
ANTERIOR
PITUITARY
TSH
THYROID
GLAND
T3 & T4
LONG-LOOP
STARVATION
FEEDBACK
GOITROGENS
T4
HIGH
SERUM
IODIDE
Endocrinopathies
Hyperthyroidism
• Level of the defect
(1, 2, 3)
• Thyrotoxicosis
• Graves’ disease
• Toxic Adenoma
• Toxic Multinodular
Goiter
• Chronic Thyroiditis
Hyperthyroidism Symptoms
•
•
•
•
•
•
•
•
•
•
•
Tachycardia/arrhythmia
Muscle tremors
Hyperreflexia
Increased core & skin
temp./ Heat Intolerant
Exophthalmos
Muscle wasting
Loose stool
Osteoporosis
Hair loss
Oligo-/amenorrhea
Irritability, Restlessness
Hypothyroidism
• 1, 2, 3
• Newborn
– Cretinism
– Lack of myelination
• Children
– Retarded Growth
– Disproportionate
Hypothyroidism
• Adult
–
–
–
–
–
Muscle weakness
Mental slowness
Tired/fatigued
Cold
Slowed intestinal
peristalsis
– Impaired renal
function
– Anemia
– Myxedema
Myxedema
Causes of
Hypothyroidism
• Iodine deficiency
• Hashimoto’s Disease
– Initially hyperthyroid
• Post-ablative
• 5’-Deiodinase Deficiency
Thyroid hormone
resistance
• Mutation of the receptor.
• Characterized by high blood levels of
both TSH and Thyroid hormones
Non-toxic goiter
• Iodine deficiency
Thyroiditis
• Subacute thyroiditis
• Chronic thyroiditis (Hashimoto’s
Disease)
– Autoimmune
Euthyroid Syndrome
• Patient presents and complains of
hypothyroid type symptoms
• Levels of Thyroid hormones (free &
total), & TSH in low normal range.
• May be due to a 5’-deiodinase
deficiency
Allopathic treatments
for thyroid disorders
• Hyperthyroidism
– Goitrogens
– Partial thyroidectomy
– Radiothyroidectomy
• Hypothyroidism
– Hormone Replacement