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Embryology• The thyroid gland arises as an outpouching
of the primitive foregut around the
third week of gestation.
• It originates at the base of the tongue at the foramen cecum
• The foramen caecum
is the vestigial remnant of the duct.
• The thyroglossal
duct develops from the median bud of the pharynx
• This initially hollow structure migrates caudally and passes in close continuity
with, and sometimes through, the developing hyoid cartilage
• Parafollicular
cells (C cells) from the neural crest reach the thyroid via the
ultimobranchial
body
• Thyroid follicles are ini ally apparent by 8 weeks, and colloid formation begins by the 11th week of gesta on.
Thyroid HistologyThe functioning unit is the lobule consists of 24–40 follicles lined with cuboidal epithelium. The follicle contains colloid in which thyroglobulin is storedThe second group of thyroid secretory cells is the C cells or parafollicular cells, which contain and secrete the hormonecalcitoninLymphaticsExtensive Anastomose
below the capsule drain to pretracheal
(Delphian)
and paratracheal
then to deep cervical and mediastinal
lymph nodes.The clinical importance of the Delphic LN, is when enlarged, becomes of
DDx
of goiter, as it moves with deglutition.
Developmental Abnormalities
• Thyroglossal
Duct Cyst and Sinus. • are the most commonly encountered congenital cervical anomalies.• During the 5th week of gesta on, duct lumen starts to obliterate, and the duct disappears by the 8th week of gesta on.
• Rarely, it may persist in whole or in part.• It may occur anywhere along the migratory path of the thyroid,
although 80% are found in juxtaposi on to the hyoid bone.
• usually asymptomatic but occasionally become infected by oral
bacteria
• Thyroglossal
duct sinuses result from infection of the cyst
secondary to spontaneous or surgical drainage of the cyst
• heterotopic
thyroid ssue present in 20% of cases.
• The diagnosis• well‐defined midline neck mass that moves upward with protrusion
of the tongue.
• thyroid scan and ultrasound have been performed to document
the presence of normal thyroid tissue in the neck.
• Treatment involves the “Sistrunk
operation,
Lingual Thyroid
• failure of the median thyroid anlage
to descend normally and may
be the only thyroid tissue present
• Intervention becomes necessary for obstructive symptoms such as
choking, dysphagia, airway obstruction, or hemorrhage.
• Medical treatment include thyroxin t4 to suppress (TSH) and
radioactive iodine (RAI) ablation followed by hormone
replacement.
• Surgical excision is rarely needed but, if required, should be
preceded by an evaluation of normal thyroid tissue in the neck to
avoid inadvertently rendering the patient hypothyroid.
Ectopic Thyroid
•Thyroid tissue has been observed adjacent to the aortic arch, within
the upper pericardium, or in the interventricular
septum.
•lateral aberrant thyroid situated lateral to the carotid sheath and
jugular vein,represents
metastatic thyroid cancer in lymph nodes
Physiology• The hormones (T3) and (T4) are bound to thyroglobulin
within the
colloid. Synthesis within the thyroglobulin
complex is controlled by
several enzymes
•1. Trapping of inorganic iodide from the blood.
•2. Oxida on of iodide to iodine.
•3. Binding of iodine with tyrosine to form iodotyrosines.
•4. Coupling of mono and di‐iodotyrosines
to form T3 and T4.
• When hormones are required, the complex is resorbed
into the cell and
thyroglobulin
is broken down. T3 and T4 are liberated and enter the
blood, where they are bound to serum proteins:albumin
• T3 is quick ac ng (within a few hours),
• whereas T4 acts more slowly (4–14 days).
THYROID HORMON FUNCTION• important for fetal brain development and skeletal maturation
• T3 increases oxygen consump on, basal metabolic rate,and heat production
• actions of catecholamines
are amplified
• increase gastrointestinal (GI) motility
• increase bone and protein turnover and the speed of muscle contraction and
relaxation
• increase glycogenolysis, hepatic gluconeogenesis, intestinal glucose
absorption, and cholesterol synthesis and degradation
TESTS OF THYROID FUNCTION
• Serum TSH (Normal 0.5–5 μU/mL)
Serum TSH levels reflect the ability of the anterior pituitary
to
detect free T4 levels There is an inverse rela onship between the free T4 level and the TSH
Total T4 levels reflect the output from the thyroid gland, whereas T3
levels are more indicative of peripheral thyroid hormone
metabolism
Thyroxine
(T4) and tri‐iodothyronine
(T3)• Only a small frac on of the total (0.03 per cent of T4 and 0.3 per cent of T3) is free and physiologically ac ve
• Total T4 levels are increased in hyperthyroid pa ents, • in those with elevated Tg
levels secondary to pregnancy
,estrogen/progesterone use, • total T4 levels decrease in hypothyroidism and in pa ents with decreased Tg levels due to anabolic steroid use and protein‐losing disorders like nephrotic syndrome.
• Free T4 (Reference Range 12–28 pmol/L) and Free T3 (3–9• pmol/L• T3 toxicity
Results of thyroid function tests in normal and pathological states.
• Thyroid functional state TSH
Free T4 Free T3
Euthyroid
Normal Normal
Normal
• Thyrotoxic
Undetectable High High
• Myxoedema
High Low Low
• Suppressive T4 therapy Undetectable High High
(often normal)
• T3 toxicity Low/undetectable Normal High
Thyroid autoantibodies
Serum levels of antibodies against thyroid peroxidase
(TPO) and
thyroglobulin
are useful in determining the cause of thyroid
dysfunction
Levels above 25 units/mL
for TPO antibody. and titres
of greater than
1:100 for antithyroglobulin
are considered significant for
autoimmune thyroiditis
.The presence of antithyroglobulin
antibody
interferes with assays of serum thyroglobulin
TSH receptor antibodies (TRAB) are often present in Graves’
disease
Thyroid imaging
Chest and thoracic inlet x‐rays
Ultrasound scanning
Computed tomography, magnetic resonance imaging and positron emission
tomography
Isotope scanning
(123I), (99mTc) Rou ne isotope scanning is unnecessary
majority (80 per cent) of ‘cold’
swellings are benign and some (5 per cent)
functioning or ‘warm’
swellings will be malignant
Fine‐needle aspiration cytology
Classification of thyroid swellings.
• Simple goiter Diffuse
Physiological
• euthyroid
hyperplastic
pupertal
• Pregnancy
MNG
• Toxic Diffuse (Graves’disease)• Multinodular
• Toxic adenomaNeoplastic
benign
malignant
• Inflammatory
Autoimmune
Chronic lymphocytic thyroiditis
• Hashimoto’s disease
• Granulomatous
De Quervain’s
• Fibrosing
Riedel’s thyroiditis
• Infective
Acute (bacterial thyroiditis
• viral, subacute
thyroiditis
• Chronic
TB,syphilis
•
• Other
AMYLOID
THYROID ENLARGEMENT
• Goitre
: generalized enlargement of the thyroid gland.
• Solitary nodule: discrete swelling in one lobe with no palpable
abnormality elsewhere
• Dominant nodule :Discrete swellings with evidence of abnormality
elsewhere in the gland
Simple goitre• Aetiology• inappropriate secretion of TSH from microadenoma in the anterior
pituitary, low level of circulating thyroid hormones
• The most important factor in endemic goitre is dietary deficiency of
iodine
• Iodine deficiency• Dyshormonogenesis defective hormone synthesis (sporadic goiters)• Goitrogens (cabbage, kale and rape), PAS, antithyroid• Thiocyanates interfere with iodide trapping
Nodular goitre
• as a result of fluctuating stimulation
• Nodules are usually multiple
• Nodules may be colloid or cellular, and cystic degeneration and
haemorrhage are common, as is subsequent calcification
• Nodules appear early in endemic goitre and later in sporadic goitre
• All types of simple goitre are more common in the female than in
the male owing to the presence of oestrogen receptors in thyroid
tissue
Diagnosis• The patient is euthyroid, • The nodules are palpable and often visible• ; they are smooth, usually firm and not hard, • the goitre is painless and moves freely on swallowing.• Hardness and irregularity, due to calcification, may simulate
carcinoma.
• A painful nodule, or rapid enlargement of a nodule raises suspicion
of carcinoma but is usually due to haemorrhage into a simple
nodule
Complications
• Tracheal obstruction• Retrosternal extension• Secondary thyrotoxicosis• Carcinoma usually follicular
Prevention and treatment of simple goitre• iodised
salt.• In the early stages, a hyperplastic
goitre
may regress if thyroxineis given in a dose of 0.15–0.2 mg daily for a few months
• nodular stage of simple goitre
is irreversibleOperationcosmetic , pressure symptoms, patient anxiety. Retrosternal
extensiondominant area of enlargement that may be neoplastic.total thyroidectomy with immediate and lifelong replacementof thyroxine