Sources of T3 - Deiodinases (T4 to T3 Conversion)
- Type 1 (Liver, Kidney, Thyroid; acts at higher T4 concentration)
- Type 2 (Pituitary, CNS; acts at lower T4 concentration)
- TSH feedback may vary due to
- Heterogenity of T3 receptors
- Heterogenity of T4 Deiodinases
- Resetting the Threshold for Negative Feedback
- Assessing Thyroid Function
- During Steady State Condition: TSH
- During Non-steady condition: TSH, Free T4
- Limitations
- "Acquired Central Hypothyroidism" or Non-Thyroidal Illness
- Low TSH and Low FT4 when sick (Way to remember: Acute Illness causes central TRH and TSH release defect - hence, Low TSH)
- TSH Transiently above normal during recovery (Way to remember: During recovery, TRH, TSH bounces back, hence, TSH transiently above normal)
- Subclinical Hyperthyroidism
- Natural History
- 323 Patients (Das et al Clin Endocrinol 2012) - 32 month follow up
- TSH (0.1 - 0.39) : 6.8 % progression
- TSH (<0.1): 20.3 % progression (Thus, much higher progression when TSH < 0.1)
- Physiological Effect of Subclinical Hyperthyroidism
- Bone:
- Decreased Bone Density
- Increased Serum Osteocalcin
- Increased Serum Hyrooyproline and Pyrrolidine links
- Heart
- Increased HR
- Increased Risk of AFib
- (28 % increased incidence of A Fib in Framingham Study with TSH < 0.1)
- Risk of A Fib in hospitalized patients
- 2 % in Euthyroid
- 13 % in Subclinical Hyperthyroid
- 14 % in Overt hyperthyroid
- Increased Contractility
- Increased LV mass Index
- Increased Intraventricular septal and posterior wall thickness
- Note: These changes prevented by B-Blocking agent
Levothyroxine - t1/2: 7 day
- Absorption: 80 %
- Miss one dose: Take 2 next day
- Miss 2 days: Take 3 next day
- Miss 4 days: Take 4 next day
- Dose: 1.6 mcg / kg
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