Trade Name(s)

  • Halycil Canadian Trade name

Ther. Class.

antithyroid agents

Pharm. Class.



  • Graves' disease with hyperthyroidism or toxic multinodular goiter in patients who are intolerant to methimazole and for whom surgery or radioactive iodine therapy is not appropriate.
  • Adjunct in the control of hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy in patients who are intolerant to methimazole.


Inhibits the synthesis of thyroid hormones.

Therapeutic Effect(s):

Decreased signs and symptoms of hyperthyroidism.


Absorption: Rapidly absorbed from the GI tract.

Distribution: Concentrates in the thyroid gland.

Metabolism and Excretion: Metabolized by the liver. Primarily excreted in the urine as metabolites.

Half-life: 1–2 hr.

TIME/ACTION PROFILE (effects on clinical thyroid status)

PO10–21 days†6–10 wkwk
†Effects on serum thyroid hormone concentration may occur within 60 min of a single dose.


Contraindicated in:

  • Hypersensitivity.

Use Cautiously in:

  • ↓ bone marrow reserve;
  • OB:  May be used during 1st trimester (consider using methimazole during 2nd and 3rd trimesters); however, fetus may develop thyroid problems; mother and fetus may be at ↑ risk for hepatotoxicity;
  • Geri:  Older adults may have ↑ sensitivity to the drug's effect; initiate therapy with lowest dose;
  • Pedi:  Children <6 yr (safety and effectiveness not established); not recommended unless methimazole not tolerated and surgery or radioactive iodine therapy not appropriate.

Adverse Reactions/Side Effects

CV: edema, vasculitis

Derm: rash, exfoliative dermatitis, hair loss, skin discoloration, STEVENS-JOHNSON SYNDROME, TOXIC EPIDERMAL NECROLYSIS, urticaria

EENT: parotitis

Endo: hypothyroidism

GI: nausea, vomiting, diarrhea, HEPATOTOXICITY, loss of taste, splenomegaly

GU: glomerulonephritis

Hemat: AGRANULOCYTOSIS, APLASTIC ANEMIA, BLEEDING, leukopenia, thrombocytopenia

MS: arthralgia, myalgia

Neuro: drowsiness, headache, paresthesia, vertigo

Resp: interstitial pneumonitis

Misc: fever, lymphadenopathy

* CAPITALS indicate life-threatening.
Underline indicate most frequent.



  • Additive bone marrow depression with  antineoplastics  or  radiation therapy.
  • Additive antithyroid effects with  lithium, or  potassium iodide.
  • ↑ risk of agranulocytosis with  phenothiazines.


PO (Adults): 100 mg every 8 hr; may be ↑ to 400 mg/day (occasional patient may require 600–900 mg/day); usual maintenance dose = 100–150 mg/day.

PO (Children  >10 yr): 50–300 mg/day given once daily or in 2–4 divided doses.

PO (Children  6–10 yr): 50–150 mg/day given once daily or in 2–4 divided doses.

Availability (generic available)

Tablets: 50 mg


  • Monitor response of symptoms of hyperthyroidism or thyrotoxicosis (tachycardia, palpitations, nervousness, insomnia, fever, diaphoresis, heat intolerance, tremors, weight loss, diarrhea).
  • Assess patient for development of hypothyroidism (intolerance to cold, constipation, dry skin, headache, listlessness, tiredness, or weakness). Dose adjustment may be required.
  • Assess patient for skin rash or swelling of cervical lymph nodes. Treatment may be discontinued if this occurs.

Lab Test Considerations:

Thyroid function studies should be monitored prior to therapy, monthly during initial therapy, and every 2–3 mo throughout therapy.

  • WBC and differential counts should be monitored periodically throughout therapy. Agranulocytosis may develop rapidly and usually occurs during first 2 mo. This necessitates discontinuation of therapy.
  • May cause increased AST, ALT, LDH, alkaline phosphatase, serum bilirubin, and prothrombin time.


  • Can be compounded by pharmacist into enema or suppository.
  • PO Administer at same time in relation to meals every day. Food may either increase or decrease absorption.

Patient/Family Teaching

  • Instruct patient to take medication exactly as directed, around the clock. If a dose is missed, take as soon as remembered; take both doses together if almost time for next dose; check with health care professional if more than 1 dose is missed. Consult health care professional prior to discontinuing medication.
  • Instruct patient to monitor weight 2–3 times weekly. Report significant changes.
  • May cause drowsiness. Caution patient to avoid driving or other activities requiring alertness until response to medication is known.
  • Advise patient to consult health care professional regarding dietary sources of iodine (iodized salt, shellfish).
  • Advise patient to report sore throat, fever, chills, headache, malaise, weakness, yellowing of eyes or skin, unusual bleeding or bruising, symptoms of hyperthyroidism or hypothyroidism, or rash to health care professional promptly.
  • Advise patient to notify health care professional of all Rx or OTC medications, vitamins, or herbal products being taken and to consult with health care professional before taking other medications.
  • Advise patient to carry identification describing medication regimen at all times and to notify health care professional of medication regimen prior to treatment or surgery.
  • Rep:  Advise females of reproductive potential to notify health care professional if pregnancy is planned or suspected or if breastfeeding.
  • Emphasize the importance of routine exams to monitor progress and to check for side effects.

Evaluation/Desired Outcomes

  • Decrease in severity of symptoms of hyperthyroidism (lowered pulse rate and weight gain).
  • Return of thyroid function studies to normal.
  • May be used as short-term adjunctive therapy to prepare patient for thyroidectomy or radiation therapy or may be used in treatment of hyperthyroidism. Treatment of 6 mo to several yr may be necessary, usually averaging 1 yr.