422 DISEASES/DISORDERS
Synthetic T 3 (liothyronine; 4–6μg/kg TID) administration is not recommended
unless impaired absorption of T 4 is suspected (very rare); thyrotoxicosis more likely
with T 3 supplementation.Levothyroxine: serum half-life 12–16 hours; peak concentration 4–6 hours after
administration.
Initial dosage of levothyroxine: 15–20μg/kg BID; this dosage is considerably higher
than standard dosages for humans and may confuse pharmacists.
Lowered initial dosage required for patients with congestive heart failure, renal
disease, diabetes mellitus, seizure conditions, and hypoadrenocortisolism; initial
dosage 10μg/kg BIDto prevent destabilization by increased metabolic rate and car-
diac demands.Thyroxine binds to soy and calcium; it should be given 1 hour before or 3 hours after
a meal.
Glucocorticoids, NSAIDS, and furosemide may increase absorption; low-dose corti-
costeroid administration may be used to increase absorption of T 4 in patients with
poor response to levothyroxine administration.Monitoring of Therapy
Initial measurement 4–6 weeks after initiation of supplementation.
Sample collection 4–6 hours after levothyroxine administration; consistency in mon-
itoring is required for proper management.
Total T 4 level should be within the normal range; reported parameters for treatment
monitoring vary by laboratory; results near the middle of the reference range pre-
ferred.Alternative method of monitoring: T 4 measurement of trough (just prior to admin-
istration of levothyroxine) and 4–6 hours post administration; trough level should
be at the low end and peak level should be at the upper end of the reference range;
particularly useful for cases supplemented only once daily.Repeat measurements 4 weeks after each dosage change.
Measurement of stable patients recommended at 6-month intervals.
TSH levels, if elevated prior to supplementation, should return to normal to low with
therapy; interpretation of adequate supplementation cannot currently be based solely
on measurement of TSH levels.Alternative Therapy Options
Twice-daily supplementation considered standard.
Once-daily supplementation may be attempted when symptoms of hypothyroidism
have resolved; if attempted, monitor for return of symptoms indicating that twice-
daily administration is required in a particular patient.Variability of drug absorption may not permit effective once-daily treatment.
Determination of success of once-daily treatment based on continued resolution of
clinical symptoms of hypothyroidism.
Intravenous L-thyroxine (4–5μg/kg BID) used to treat hypothyroid crisis (myxedema
coma).