Pharm
Antithyroid Drug
search
Antithyroid Drug
, Thioamide, Thionamide, Propylthiouracil, PTU, Methimazole, Tapazole
See Also
Hyperthyroidism
Hyperthyroidism Management
Indications
Hyperthyroidism
in children and adolescents
Pregnancy (PTU)
Severe
Grave's Disease
(e.g.
Thyroid Storm
)
Subclinical Hyperthyroidism
Mechanism
Thionamides block T3 and T4 synthesis (by inhibiting
Thyroid
peroxidase)
Dose
Grave's Disease
Target dose to
Free T4
high end of normal range
Methimazole (Tapazole)
Preferred first-line agent
Avoid in first trimester due to congenital defect risk
Esophageal atresia
Choanal Atresia
Aplasia cutis
Abdominal wall defects
Ventricular Septal Defect
Advantages over Propylthiouracil (PTU)
Once daily dosing
Decreases T4 and T3 levels more rapidly
Much safer than PTU
Lower risk of
Agranulocytosis
at moderate doses
Does not carry the same liver toxicity risk as PTU
Dosing:
Hyperthyroidism
Start: 10-30 mg PO qd
Titrate Methimazole dose down after 4-6 weeks
Goal: maintain normal
Thyroid
function
Dose: reduce to 5-10 mg per day
Dosing: Other
Subclinical Hyperthyroidism
: 5 mg PO qd
Propylthiouracil (PTU)
Indications (reasons to use PTU instead of Methimazole)
Lactation
Pregnancy (both PTU and Methimazole are Category D)
Use PTU in first trimester
Use Methimazole in second and third trimester
Dosing:
Hyperthyroidism
Start: 100 mg po tid
Maximum: 150 mg every 6-8 hours
Titrate PTU dose down after 4-6 weeks
Goal: maintain normal
Thyroid
function
Dose: reduce to 50-100 mg per day
Dosing: Other
Thyroid Storm
: 1 gram load, then 300 mg PO q6 hours
Subclinical Hyperthyroidism
: 50-100 mg PO qd
Adverse effects (3 per 1000 patients)
Drug-Induced Agranulocytosis
(more common with PTU)
Risk increases over age 40 years
Occurs in 0.17% of those on Methimazole
Presentation: Infection (e.g.
Pharyngitis
)
Course
Resolves within 2-3 weeks after drug stopped
Severe, refractory course may occur
Contraindication to further Antithyroid Drugs
Jaundice
Cholestatic
Jaundice
Occurs more commonly with Methimazole
Hepatitis
Occurs with both Methimazole and Propylthiouracil (PTU)
Methimazole associated hepatitis
Incidence
: 3.17 per 1000 person-years
Acute Hepatic Failure
: 0.32 per 1000 person-years
Propylthiouracil (PTU) is associated with severe liver injury
Adults: 1 in 10,000
Children: 1 in 2,000
Vasculitis
Lupus-Like Syndrome
Aplastic Anemia
Rash or
Pruritus
Switch from PTU to Methimazole if severe
Arthralgia
or
Polyarthritis
Fever
Congenital abnormalities possible with Methimazole
Monitoring
Repeat examination every 3 months
Pregnancy Test
before starting therapy
Lab Testing while on antithyroid medications: Monthly for 6 to 12 months
Complete Blood Count
(CBC)
Obtain at baseline
Repeat if
Pharyngitis
or fever occur (need not be done without symptoms or signs)
Liver Function Test
s
Obtain at baseline and if symptoms develop
Thyroid Function Test
s (baseline, then every 4-8 weeks, then after stabilizing, every 3 months)
Thyroid Stimulating Hormone
(TSH) after stabilizing, once on every 3 month schedule
Free T4
Free T3
Lab Testing after completing antithyroid medications
Obtain
Thyroid Function Test
s every 1-3 months for 6-12 months
Indications to discontinue medication (via taper)
Total treatment course of 12-18 months AND
TSH normalized for 6-12 months
Indications to consider
Thyroid
ablation
Inadequate suppression at 12 months from initiation
Efficacy
Predictors of Relapse (occurs in 30-70% of cases within first year)
Tobacco Abuse
Large
Goiter
Thyroid
Stmulating
Antibody
high at end of treatment
References
Kravets (2016) Am Fam Physician 93(5): 363-70 [PubMed]
Type your search phrase here