What is propylthiouracil, and how does it work (mechanism of action)?
Propylthiouracil (PTU) is an oral medication
that is used to manage hyperthyroidism which is due to an overactive thyroid
gland. It is an anti-thyroid drug that has a mechanism of action that is similar
to methimazole (Tapazole).
Graves’ disease is the most common cause of
hyperthyroidism. It is an autoimmune disease where an individual makes
antibodies to thyroid stimulating hormone receptors on cells of the thyroid
gland and then trigger overproduction of thyroid hormone by the cells. The two
thyroid hormones manufactured by the thyroid gland, thyroxine (T4 ) and
triiodothyronine (T3), are formed by combining iodine and a protein called
thyroglobulin with the assistance of an enzyme called peroxidase. PTU inhibits
iodine and peroxidase from their normal interactions with thyroglobulin to form
T4 and T3. This action decreases production of thyroid hormone. PTU also
interferes with the conversion of T4 to T3, and, since T3 is more potent than
T4, this also reduces the activity of thyroid hormones. The FDA approved PTU in
July 1947.
What brand names are available for propylthiouracil?
None available
Is propylthiouracil available as a generic drug?
GENERIC AVAILABLE: Yes
Do I need a prescription for propylthiouracil?
Yes
What are the side effects of propylthiouracil?
The most common side effects are related to the skin, such as:
Other important side effects include:
- swelling,
- nausea,
- vomiting,
- heartburn,
- loss of taste,
- joint or muscle aches,
- numbness, and
- headache.
Less common but serious side
effects have occurred with PTU therapy. A decrease of white blood cells in the
blood (agranulocytosis) may occur. Symptoms and signs of agranulocytosis include
infectious lesions of the throat, the gastrointestinal tract, and skin with an
overall feeling of illness and fever.
A decrease in blood platelets
(thrombocytopenia) also may occur. Since platelets are important for the
clotting of blood, thrombocytopenia may lead to excessive bleeding. Severe
liver
injury and acute liver failure, in some cases fatal, have been associated with PTU. Some adults and pediatric patients required liver transplantation.
What is the dosage for propylthiouracil?
The initial adult dose of PTU is 300 mg/day divided into 3
divided doses. PTU is usually administered every eight hours. Patients with
large goiters may require initial doses up to 900 mg daily. The usual long-term
adult dose after initial treatment is 100-150 mg/day.
Which drugs or supplements interact with propylthiouracil?
PTU may increase the effect of oral blood thinners,
for example warfarin (Coumadin). Therefore, warfarin dose changes and monitoring
for the effects of warfarin on blood clotting are necessary.
Hyperthyroidism may
cause increased elimination of beta blockers (for
example, propranolol
[Inderal, Inderal LA, Innopran XL)). Once
hyperthyroidism is reversed the excretion of beta-blockers may return to normal
and less beta-blocker will be needed to provide the same effect.
Digoxin
(Lanoxin) blood
levels may be increased when hyperthyroidism is reversed in patients on a stable
digoxin dose. A smaller dose of digoxin may be needed in order to avoid toxicity
of digoxin. Similarly, theophylline elimination may decrease when
hyperthyroidism is reversed in patients on a stable theophylline dose. A reduced
dose of theophylline may be needed in order to avoid toxicity of theophylline.
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Is propylthiouracil safe to take if I’m pregnant or breastfeeding?
PTU crosses the placenta and may cause harm to the fetus.
If it is necessary to use PTU during pregnancy the lowest effective dose should
be used. Since methimazole is associated with fetal abnormalities, PTU is used
during the first trimester if an antithyroid drug is needed.
PTU is excreted in
breast milk in small amounts.
What else should I know about propylthiouracil?
What preparations of propylthiouracil are available?
Tablet: 50 mg.
How should I keep propylthiouracil stored?
PTU should be stored at room temperature, 15 C – 30 C (59 F – 86 F).