Antithyroid Drugs
The thioureylenes are compounds
belonging to the family of thioamides, and the most important representative of
the group is propylthiouracil (PTU). Other examples include methima-zole and
carbimazole, which are organic thiourea antithyroid drugs included in the
chemical class of mercaptoimidazolines. Carbimazole is a prodrug of
methimazole. Carbimazole is rapidly and completely metabolised to methimazole
in the body, with the antithyroid activity of carbimazole dependant upon this
conversion to methimazole.
Antithyroid drugs inhibit the
formation of thyroid hormones by interfering with the incorporation of iodine
into tyrosyl resi-dues of thyroglobulin. They also inhibit the coupling of
these iodotyrosyl residues to form iodothyronines by inhibiting the peroxidase
enzyme.
In addition to blocking hormone
synthesis, propylthiouracil (unlike other antithyroid drugs) inhibits the
peripheral deiodin-ation of thyroxine to tri-iodothyronine.
Propylthiouracil, carbimazole, and
methimazole are used in the management of hyperthyroidism including the
treatment of Graves’ disease, thyroid storm, and in preparing individuals for
thyroidectomy and as an adjunct to radioiodine therapy.
·
Agranulocytosis: Chronic ingestion
of PTU, carbimazole, or methimazole has been reported to cause leukopenia,
agranulocytosis, aplastic anaemia, eosinophilia, leukaemia, thrombocytopenia
and hypoprothrombinaemia.
·
Hepatotoxicity: Chronic ingestion of
PTU produces a characteristic hepatocellular or mixed cytotoxic-cholestatic
hepatitis, believed to be a hypersensitivity reaction.
·
Arthralgia: Arthritis syndrome,
which is rare, is generally transient, occurring within 2 months of initiation
of therapy and resolving within 4 weeks of stopping therapy.
·
Following therapeutic use of
carbimazole or methimazole, dizziness, paresthesias, and headache have been
described occasionally.
·
Skin rashes may occur secondary to
haematologic toxicity during chronic therapy.
·
Gastric upsets.
·
Methimazole therapy has uncommonly
been reported to be related to the occurrence of T-lymphotropic virus type
I-associated uveitis, with vitreous opacities and retinal vasculitis. The exact
mechanism of this adverse reaction is unknown.
·
Reversible nephrotic syndrome has
been observed in a very few patients during therapy with methimazole or
carbimazole. It is likely that this is the result of a direct toxic effect on
the glomerular basement membrane and epithelial podocytes.
·
Carbimazole hypersensitivity
resulting in the development of antibodies to connective tissue or neural
antigens in the cochlea has been reported. Hearing loss and tinnitus resulted.
·
Vomiting, headache, fever,
arthralgia, pruritis and pancy- topenia.
·
Clinical signs/symptoms of overdose
with carbimazole or methimazole may include manifestations of hypothy- roidism,
including nausea and vomiting, constipation, headache, drowsiness, coldness,
dry and puffy skin, muscle aches, and goitre. CNS depression and/or stimulation
may occur. Hypothyroidism is unlikely to develop after a single acute overdose
ingestion. Less frequently, overdose may result in hepatic enzyme changes or
neuropathies.
·
Very little data are available on
the effects of acute over- dose with propylthiouracil. Decreased T3 and
elevated alkaline phosphatase levels were the only effects seen after a massive
overdose in a young girl.
·
Chronic overdose may result in
clinical hypothyroidism (nausea and vomiting, constipation, headache,
drowsiness, coldness, dry and puffy skin, muscle aches). Propylthiouracil is
transferred across the placenta and can induce goitre and hypothyroidism in the
unborn. Hyperthyroidism may also occur as a compensatory mechanism. Infants of
women with Graves’ disease who were treated with propylthiouracil may be either
hypothyroid or hyperthyroid.
·
The administration of
propylthiouracil to the mother from the 14th week of pregnancy or later has
been of concern because of the possible development of goitre and mental
retardation in the infant. Incidence of birth defects was not significantly
higher in children of women who had been treated with propylthiouracil during
pregnancy.
·
Methimazole crosses the placental
membrane readily and can induce goitre and cretinism in the developing foetus.
Congenital defects such as aplasia cutis (manifested by scalp defects),
oesophageal atresia with tracheoesophageal fistula, and choanal atresia with
absent/hypoplastic nipples have occurred rarely in infants exposed to
methimazole in utero. Chronic ingestion of PTU, carbimazole or methimazole can
result in a variety of immunologic-mediated adverse effects which are not
expected to occur in overdose, including agranulocytosis, aplastic anaemia,
vasculitis, lupus-like syndrome, and hepatitis. Fatal hepatic necrosis has been
reported. Most adverse effects are dose-related and occur within the first 4 to
8 weeks of therapy.
·
Chronic propylthiouracil use has
been linked with acute myeloblastic leukaemia in isolated cases.
·
Symptomatic and supportive measures.
·
Alkaline phosphatase levels have
been elevated in overdose cases involving PTU. Isoenzyme determination is
recom- mended. Elevations in alkaline phosphatase do not neces- sarily reflect
liver toxicity and may be related to increased bone or bile isoenzyme.
·
Monitor thyroid function tests and
liver function in symp- tomatic patients.
·
Total and differential leukocyte
counts should be performed in patients with suspected haematologic reactions.
Agranulocytosis is usually reversible on discontinua-tion of therapy.
Administration of recombinant humangranulocyte colony-stimulating factor may
hasten recovery.Granulocyte-macrophage colony-stimulating factor(GM-CSF) and
granulocyte colony-stimulating factor(G-CSF), or filgrastim, may be effective
in acceleratingbone marrow recovery after carbimazole or methimazole therapy.
Erythrocyte and platelet transfusions may be necessary. Treatment with these
drugs has been shownto significantly shorten recovery time in patients
withmethimazole-induced agranulocytosis.
·
Infection or fever in neutropenic
patients should be treated aggressively with antibiotics. Cultures and
sensitivities should be done. Appropriate broad-spectrum antibiotics should
probably be initiated before culture results are known. Adjust antibiotic regimen based on
culture results.
·
Haemodialysis, peritoneal dialysis,
forced diuresis, or char- coal haemoperfusion have not been shown to be
beneficial in overdose with these
agents.known. Adjust antibiotic regimen based on culture results.
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