1. Inhibition of new
hormone synthesis with thiouracils (PTU) or imidazoles (methimazole and carbimazole)
PTU blocks the enzyme thyroidal peroxidase to inhibit thyroid
hormone synthesis. It is generally preferred over methimazole because it also
inhibits the peripheral conversion of T4 to T3.
The dose of PTU is a 500-1000 mg loading dose, followed by 250 mg every
4 hours. Methimazole is dosed 60-80 mg/day
in divided doses.
2. Blockade of
release with iodine or lithium
Blocking release of
thyroid hormone is best accomplished with iodine, but lithium can be used in
iodine-allergic patients. It is important
not to administer iodine until after the synthetic pathways have been blocked
with PTU (at least 30 minutes), otherwise administration of iodine might
cause more thyroid hormone to be formed.
3. Inhibition of
peripheral effects with β-adrenergic agents
Propranolol is the preferred treatment to block peripheral
effects of thyroid hormone because of its non-selective effects and the
additional benefit of inhibiting peripheral conversion of T4 to T3. If a
contraindication to propranolol exists (e.g. asthma, congestive heart failure),
then a selective agent such as esmolol may be used.
4. Inhibition
of Enterohepatic Circulation of Thyroid Hormone
Thyroid
hormone is metabolized in the liver where it is conjugated to glucuronides and
sulfates and excreted into the intestine in bile, while unconjugated free
hormones are reabsorbed into circulation.
Cholestyramine, by binding conjugated products, promotes their excretion,
thereby lowering thyroid hormone levels. The recommended dosing regimen is 1-4
g twice daily.
5. Additional
treatment considerations
Inciting events
should be addressed (e.g. infection, DKA, trauma, etc.). Administration of glucocorticoids is
recommended because thyroid storm can precipitate adrenal crisis (relative
adrenal insufficiency), with similar dosing regimens. Dexamethasone may be
preferred, as it also blocks the peripheral conversion of T4 to T3. Antipyretics and external cooling methods may
be considered, but salicylates should be avoided as they may increase free
hormone levels. Most patients will benefit from IV fluids to replace
significant volume loss from hyperthermia and GI losses. Patients failing medical therapy may require surgical
intervention.