congenital hypothyroidism thyroid gland embryology thyroid hormone synthesis feedback mechanisms...
TRANSCRIPT
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Congenital Hypothyroidism
• Thyroid gland embryology• Thyroid hormone synthesis• Feedback mechanisms• In-utero + neonatal dynamics• Etiology• Manifestations• Treatment• Prognosis
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Thyroid Embryology
Median anlage – pharyngeal floor
Lateral anlagae 4th pharyngeal pouch
Fusion of both parts
Migration to anterior neck (by ED50)
Thyroid transcription factors:
TTF-1, TTF-2, PAX-8
Responsible for less than 10% of CH
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Thyroid Migration
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The Thyroid gland
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Thyroid Hormone Biochemistry
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Production of thyroid hormones
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Thyroid Hormone Synthesis
• Iodide trapping
• Synthesis of thyroglobulin
• Organification of iodotyrosine
• Coupling, storage of T3 and T4 in colloid
• Endocytosis of colloid droplets
• Hydrolysis of TG to MIT, DIT, T3 and T4
• Secretion and circulation
• Deiodination of MIT and DIT, Iodine recycling
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Protein binding, receptor affinity
• Binding proteins -
TBG, Albumin and Prealbumin
• Free T4 – normal serum levels -10-20pmol/l
• Free T3 - normal serum levels -3-7 pmol/l
• T3 affinity to TR - X10 T4 affinity
• T3 most active thyroid hormone
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Monodeiodinases
• MDI - T4 to T3 in peripheral tissues
• MDII - T4 to T3 in brain, pituitary
• MDIII - T4 to rT3 - many tissues,
abundant in fetus and placenta
• 80% of T3 - from peripheral conversion
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Allan-Herndon-Dudley syndrome
Described -1944, molecular description- 2003
Muscle hypotonia and hypoplasia Intellectual impairment Caused by mutation in SLC16A2/MCT8 Lack of T3 transport to the brain Normal T4 transport ---The brain needs T3
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Allan-Herndon-Dudley syndrome - IQ in 26 patients
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Fetal and newborn thyroid function
• Fetal pituitary and thyroid - 10-12 wks.
relatively inactive
• From midgestation increased TSH and T4
• T3 low throughout gestation (low MDI)
• rT3 - high by 20-24 wks (high MDIII),
declines after birth at 2-3 wks to adult levels
• After delivery - TSH, T4 and T3 surges
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Thyroid Hormone Levels after Birth
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Control of Thyroid Hormone Secretion
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Thyroid Hormone Effects
• Brain development in infancy
• Somatic growth and development
• Thermogenesis
• Adrenergic effects
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Transient dysfunction - preterm
• Transient hypothyroxinemia
- in 50% before 30 wks.
- normal TRH response
- hypothalamic immaturity
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Transient dysfunction – preterm (2)
Transient primary hypothyroidism
- normal TSH and T4 at birth
- later T4 decreases and TSH increases
- causes - Iod. deficiency, Iod. solutions
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Transient dysfunction – preterm (3)
• Low T3
- Delayed, reduced TSH and T4 surge- Delayed T3 increase- Severe cases - also low T4 and TSH
Etiology: inhibition of MDI by - undernutrition, hypoxia, hypoglycemia, sepsis, hypocalcemia, birth trauma
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Congenital Hypothyroidism
• Incidence
• Worldwide 1:4,000-1:3,000
• F>M - 2:1
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Congenital Hypothyroidism
• Etiology
• ectopic gland 42-48%
• athyreosis 29-35%
• dyshormonogenesis 22-25%
• all others < 0.1%
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TTF-2 mutation
Spiky hair, hypertelorism, micrognathia, cleft palatePark SM, Chatterjee VK. J Med Genet 2005;42:379-89
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Lingual thyroid
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Radionuclide scan (Tc99) of thyroid
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Congenital Hypothyroidism
• Other causes
• maternal iodine deficiency (“endemic”) •TRH/TSH deficiency
- isolated: familial, sporadic- in panhypopituitarism
•Thyroid hormone resistance
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Congenital HypothyroidismManifestations
• Few in 1st 6-12 wks.
• Early - prolonged jaundice - poor feeding
- transient hypothermia - large post.
fontanelle
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Congenital Hypothyroidism Late Manifestations
• Thickened tongue• Hoarse cry • Hypotonia• “Potbelly”• Constipation• Bradycardia, • Low BP • MENTAL RETARDATION
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Congenital Hypothyroidism - Untreated
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Congenital Hypothyroidism- screening
• Logic - prevention of retardation• Method
- whole blood, filter paper, - 3rd day of life- logistics of reporting
• In Israel - first T4, if low – TSH (except for preterm)• USA - first TSH, if high - T4
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Heel-prick method for screening
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Guthrie paper
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Congenital Hypothyroidism
• Repeat tests and start treatment
• Thyroid imaging scan
• 10-15 mg/kg l-thyroxine
• assure compliance
What to do with +ve screen?
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Shortcomings of screening methods
Primary T4 screen False positives – TBG deficiency False negatives – early test, T4 can be normal
Primary TSH screen False positives – early test, delayed decline False negatives –
delayed TSH rise2nd/3rd hypothyroidism
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Follow-up
• Serum levels of TSH FT4 and T3 (or FT3)
• Growth
• Bone Age
• Note compliance before adjusting dose
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Addition of T3 treatment
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Addition of T3 treatment
Strich D, Neogolni L, Gillis D, JPE&M ,2013
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• Worse if athyreosis (in utero hypothy)
• Worse if mother hypothyroid
• Usually normal intelligence if RX early
• Significant mental impairment in
screened false negatives
Prognosis
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