Flashcards in Biochemistry Thyroid Deck (55)
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Thyroid Hormone
T3 is 5x more active as T4
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T4 secretion
T3 secretion
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T3 secretion
<20% secreted from thyroid, majority from peripheral de-iodination of T4.
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3 proteins which bind thyroid hormone
Thyroid binding globulin (TBG)
Thyroid binding pre-albumin
Albumin
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Bound T4 proportions
• 99.98% of T4 is bound (70% TB, 20% TBPA, - 10% Alb
• 99.7% T3 is bound
• Only the free forms (unbound are active)
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Patient 1: serum results
T.S.H 60.0
Free T4 4.4
Free T3 ---
Clinical information: tired, weight gain
Diagnosis
1o hypothyroidism – impaired T4 production and loss of negative feedback
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Primary Hypothyroidism: Biochemical Features
Raised TSH
Low Ft4
Ft3 – not helpful
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Primary Hypothyroidism:Clinical Features
Lethargy, tiredness
Weight gain
Cold intolerance
Coarsening of hair and skin
Slow reflexes, hoarseness
Constipation
Menstrual abnormalities
Bradycardia
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Patient 2 →
T.S.H -10.0 (abnormal)
Free T4 – 13.2 (decreased slightly but not abnormal)
Free T3 ---
Clinical Information
Cold intolerance, constipation
Diagnosis
Compensated Hypothyroidism
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Compensated Hypothyroidism: Biochemical Features
Raised TSH (4-15 min/L)
Low normal FT4
+ve Anti-thyroid peroxidase antibodies
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Compensated Hypothyroidism: Wickham study
Management of this form of hypothyroidism is looking at the annual risk of overt hypothyroidism of TSH >6.0min/L and or TPOAb positive – thyroxine guidelines → TSH +10 even if T4 is low/normal.
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Patient 3 →
T.S.H – 10
Free T4 ---
Free T3 ---
Clinical information: On Thyroxine
Diagnosis
This patient requires increased dose to suppress TSH
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Thyroxin Replacement → Aiming for levels of T.S.H and FT4 of
Ideal – adequate replacement
T.S.H – 2.0 (0.3-4.0)
Free T4 of 16.8 (10.0-24.0)
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Thyroxin Replacement → Inadequate replacement – low dose or poor compliance
T.S.H – 20.0
Free T4 – 5.0
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Thyroxin Replacement → Irregular compliance or recent change in dose (need to be on stable dose for 6 weeks)
T.S.H 12.0
Free T4 16.8
→ Under replaced but normal = adequate therefore need to take results after 6 weeks
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Thyroxin Replacement → Adequate/over replacement increased risk AF (not normal to measure FT3)
T.S.H <0.02 (suppressed)
Free T4 – 23.0
Free T3 – 3.0
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Thyroxin Replacement →Over replacement
T.S.H <0.02
Free T4 34.0
Free T 3 ---
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Thyroxin Replacement → Special situations
Patients with thyroid cancer on thyroxine
Some patients may be given T3 (deliberately supress TSH)
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Thyroxin Replacement → Using T3 for replacement is difficult because
• More potent
• Increased risk
• T0.5 shorter therefore management and dosing difficult
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Patient 4 →
T.S.H <0.02
Free T4 50.2
Free T3 22.0
Clinical Information: Weight loss, palpitations
Diagnosis
Thyrotoxicosis
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Primary Hyper-thryoidism: Biochemical Features
Undetectable TSH
Raised FT4
Raised Ft3
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Primary Hyper-thryoidism: Clinical Features
Weight los
Heat intolerance
Palpitations
Agitation, tremor
Muscle Weakness
Diarrhoea
Thyroid eye disease
Menstrual abnormalities
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Best discrimination of hyperthyroidism
FT3 then FT4
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T3 toxicoisis serum levels
T.S.H <0.02
Free T4 23.0
Free T3 12.0
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T3 toxicoisis diagnosis requires
Need FT3 measurement in patients with high normal – slight increased FT4
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Not Clear thyrotoxicosis
T.S.H <0.02 (low)
Free T3 23.0 (normal/high)
Free T4 6.8 (normal)
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Not Clear thyrotoxicosis management
Recheck in recouple of months to confirm
Remains suppressed (due to risks) then therefore confirm anti-thyroid treatment but evidence not 100% clear.
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Effects of non-thyroidal illness→
• Common in hospitalised patients
• Occurs in a variety of pathological conditions
• Pattern of results
o TSH low (may be increased in recovery phase)
o FT4/FT3 low or normal – metabolic response to illness
o Increased reverse T3 – metabolically inactive
→ Basal metabolism slowed down to protect body from illness.
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Causes of TSH suppression
• TNF
• IL-1
• Low TRH
• Somatostatin
• Glucocorticoids
Dopamine
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