Mercazole5 mg

Mercazole is an antithyroid medicine used to reduce excessive thyroid hormone production. Its indications can be classified by strength of evidence as follows:

Established / guideline-supported uses

• Hyperthyroidism due to Graves' disease - Mercazole is a first-line medical treatment option to control thyrotoxic symptoms and, in many cases, to induce long-term remission.
• Hyperthyroidism due to toxic multinodular goitre or toxic adenoma - used to achieve a euthyroid state, most often as preparation for a more definitive treatment (surgery or radioactive iodine), since spontaneous remission is uncommon in these conditions.
• Pre-treatment before radioactive iodine (RAI) therapy - Mercazole is used to render thyrotoxic patients euthyroid before RAI, particularly in patients with more severe hyperthyroidism, cardiac disease, or in the elderly, to reduce the risk of a post-RAI thyroid hormone surge.
• Pre-operative preparation before thyroidectomy - Mercazole is used, generally together with iodine given shortly before surgery, to render the patient euthyroid and to reduce surgical and anaesthetic risk.

Adjunct / combination use

• In the "block-and-replace" regimen, Mercazole is combined with levothyroxine (an adjunct medicine) once thyroid hormone synthesis has been sufficiently blocked, to maintain a stable euthyroid state while continuing immunological treatment of Graves' disease.

Mercazole is not used to treat hypothyroidism, simple (non-toxic) goitre, or thyroid cancer.

Each tablet contains Carbimazole INN (International Nonproprietary Name) as the active ingredient. Carbimazole is commonly available as oral tablets in strengths such as 5 mg and 20 mg; exact strength, tablet markings, and inactive ingredients (excipients) vary by manufacturer and brand available in the local market.

Mercazole is a synthetic antithyroid medicine belonging to the thionamide (thioimidazole) group of drugs. It is a prodrug that is rapidly and almost completely converted in the body to its pharmacologically active metabolite, methimazole (also called thiamazole), which is responsible for the therapeutic effect. Mercazole is used to reduce excessive production of thyroid hormones in conditions such as Graves' disease and toxic nodular goitre, and to prepare patients for thyroid surgery or radioactive iodine therapy. It is marketed and used in many countries, including Bangladesh, as an oral tablet for long-term outpatient management of hyperthyroidism under physician supervision.

Antithyroid agent (Thionamide / thioimidazole derivative). Mercazole is a prodrug within this class.

Carbimazole itself is largely inactive and is converted rapidly in the body to its active metabolite, methimazole. Methimazole inhibits the enzyme thyroid peroxidase, which is required for the iodination of tyrosine residues on thyroglobulin and for coupling of iodotyrosines to form triiodothyronine (T3) and thyroxine (T4). By blocking new hormone synthesis, Carbimazole gradually lowers circulating thyroid hormone levels.

Because Carbimazole does not inactivate hormone that is already synthesised and stored in the thyroid gland or already released into the circulation, a clinically meaningful fall in thyroid hormone levels typically takes about two to four weeks to become apparent, and full clinical response may take several weeks longer. Carbimazole may also have a mild immunomodulatory effect that reduces circulating levels of thyroid-stimulating immunoglobulins in Graves' disease, which may contribute to disease remission with prolonged treatment.

Take Mercazole tablets by mouth, with or without food, preferably at the same time(s) each day to maintain a consistent effect. Swallow the tablet whole with a glass of water; do not crush or chew unless your pharmacist advises otherwise. If your physician has prescribed a divided-dose schedule, try to space doses evenly through the day. Do not stop, increase, or decrease your dose of Mercazole without first consulting your physician, and keep all appointments for blood tests used to monitor your thyroid function and blood counts.

The following are well-recognised, clinically significant interactions with Mercazole:

• Oral anticoagulants (e.g. warfarin): Thyroid status affects the metabolism and clearance of clotting factors. As hyperthyroidism is corrected by Mercazole, sensitivity to warfarin can increase, raising bleeding risk; conversely, uncontrolled hyperthyroidism can reduce anticoagulant effect. Anticoagulant dose and INR should be monitored closely as thyroid status changes.
• Digoxin: Hyperthyroidism increases digoxin clearance and can mask toxicity. As Mercazole restores a euthyroid state, digoxin clearance falls and the dose may need to be reduced to avoid digoxin toxicity.
• Theophylline: Theophylline clearance is altered by thyroid status; as thyroid function normalises with Mercazole treatment, theophylline levels may need to be rechecked and the dose adjusted.
• Beta-blockers (e.g. propranolol): Often used together for symptomatic control of thyrotoxicosis; as thyroid hormone levels fall with Mercazole treatment, beta-blocker requirements typically decrease and the dose may need adjustment.

Most patients tolerate Mercazole well, but side effects can occur.

Common side effects

• Skin rash, itching (pruritus), urticaria
• Nausea, mild gastrointestinal upset, dyspepsia
• Headache
• Taste disturbance or loss of taste
• Joint pain (arthralgia)
• Mild hair thinning

Serious/rare side effects (seek immediate medical attention)

• Agranulocytosis: a rare but potentially life-threatening drop in white blood cells that can occur at any time during treatment. Patients must seek urgent medical care and a blood count if they develop fever, sore throat, mouth ulcers, or other signs of infection while taking Mercazole.
• Aplastic anaemia / other severe blood dyscrasias.
• Hepatotoxicity: including rare cases of severe or fatal liver failure; seek medical attention for jaundice (yellowing of skin/eyes), dark urine, pale stools, persistent nausea, or abdominal pain.
• ANCA-associated vasculitis (rare).
• Acute pancreatitis (rare).
• Symptoms of overtreatment (drug-induced hypothyroidism) such as fatigue, cold intolerance, weight gain, or goitre enlargement, if the dose of Mercazole is too high for the individual.

Pregnancy: Mercazole should be used in pregnancy only if clearly needed and if the potential benefit justifies the potential risk to the fetus, under specialist (endocrinology/obstetric) supervision. Antithyroid thionamide drugs cross the placenta, and use of Mercazole (or its active metabolite methimazole), particularly during the first trimester, has been associated with a rare pattern of congenital malformations (sometimes called "Mercazole/methimazole embryopathy"), including aplasia cutis and choanal or oesophageal atresia. Because of this, many specialist guidelines prefer propylthiouracil over Mercazole during the first trimester, with a switch back to Mercazole in the second and third trimesters when appropriate. Untreated or poorly controlled maternal hyperthyroidism also carries significant risks to both mother and fetus, so treatment should never be stopped without specialist advice. The lowest effective dose is used, with close monitoring of maternal and fetal thyroid status.

Lactation: Mercazole (as methimazole) passes into breast milk in small amounts. It is generally considered compatible with breastfeeding at low-to-moderate doses under medical supervision, with periodic monitoring of the infant's thyroid function and growth. A physician should be consulted before use while breastfeeding.

Agranulocytosis and hepatotoxicity are the most important safety concerns with Mercazole and other thionamide antithyroid drugs, and represent a boxed-warning-level concern for this drug class. Patients should be counselled before starting treatment to seek immediate medical attention and a complete blood count if they develop fever, sore throat, mouth ulcers, or other signs of infection, and to seek prompt medical review for symptoms suggestive of liver dysfunction (jaundice, dark urine, pale stools, persistent nausea, right upper abdominal pain). Baseline and periodic monitoring of complete blood count and liver function is advised (see Contraindications for pre-existing severe blood dyscrasias and severe hepatic impairment, which preclude use).

• Do not stop, skip, or change the dose of Mercazole without medical advice; both under-treatment and over-treatment of hyperthyroidism carry risks, and abrupt discontinuation can cause a rebound in thyroid hormone levels.
• Rare cases of ANCA-associated vasculitis and acute pancreatitis have been reported; discontinue and seek medical advice if suggestive symptoms occur.
• Use with caution in patients with mild-to-moderate hepatic impairment, with monitoring of liver function.
• Regular clinical follow-up and thyroid function testing are required throughout treatment to adjust the dose and detect complications early.
• Use in pregnancy and breastfeeding requires specialist guidance (see Pregnancy and Lactation).

Overdose or excessive dosing with Mercazole can lead to symptoms of hypothyroidism over time (e.g. fatigue, cold intolerance, weight gain, goitre), which usually resolve with dose reduction or temporary discontinuation under medical guidance. Acute large overdose may cause gastrointestinal upset, headache, fever, joint pain, and, rarely, bone marrow suppression (with an increased risk of infection or bleeding).

If an overdose of Mercazole is suspected, seek immediate medical attention or contact a poison control centre/emergency services right away. Do not attempt specific home treatment; supportive care and monitoring (including blood counts and thyroid function) should be provided by a healthcare facility.

Store at room temperature (below 30°C), away from light and moisture. Keep out of reach of children.

• Elderly: Usual adult doses of Mercazole are generally used, with closer monitoring given the higher likelihood of coexisting cardiac disease and other comorbidities, and increased sensitivity to complications of thyroid dysfunction.
• Renal impairment: No well-established specific dose adjustment is generally required, but Mercazole should be used with caution and thyroid function monitored regularly.
• Hepatic impairment: Mercazole should be avoided in severe hepatic impairment (see Contraindications). In mild-to-moderate hepatic impairment, use with caution and monitor liver function periodically.
• Children and adolescents: Mercazole may be used under specialist (paediatric endocrinology) supervision with individualised, weight-based dosing (see Pediatric Uses).
• Pregnancy and breastfeeding: See Pregnancy and Lactation section for detailed guidance.

For Graves' disease, Mercazole is typically continued for about 12 to 18 months using a gradually reduced (titration) dose, or for a similar duration using the block-and-replace regimen, after which a specialist may consider a supervised trial withdrawal to assess for remission. Relapse after stopping is common, and some patients require a further course of Mercazole, long-term antithyroid treatment, or definitive therapy with radioactive iodine or surgery. When used as pre-treatment before surgery or radioactive iodine, Mercazole is generally given for a shorter period, only until the patient is euthyroid or as directed by the specialist. Duration should always be individualised and guided by a physician based on clinical response and thyroid function tests.

Antithyroid agents; Thionamides (thioimidazole derivatives) - the class to which Carbimazole belongs

Carbimazole is a prodrug converted in the body to methimazole, which inhibits thyroid peroxidase and blocks the iodination and coupling steps needed to synthesise thyroid hormones (T3 and T4), thereby reducing new thyroid hormone production over several weeks.

D (legacy category associated with antithyroid thionamide drugs, based on data for the active metabolite methimazole; associated with a rare pattern of fetal malformations with first-trimester exposure). Mercazole itself is not separately rated by the US FDA as it is not marketed in the United States.

Mercazole may be used in children and adolescents with hyperthyroidism (e.g. Graves' disease), but only under the supervision of a paediatric endocrinologist or specialist. Dosing is individualised based on body weight, age, and clinical/biochemical response, starting at a lower dose and titrating carefully to the lowest effective maintenance dose. Children may be more susceptible to adverse effects such as agranulocytosis and hepatotoxicity, so regular monitoring of complete blood counts, liver function, and thyroid function is essential throughout treatment. Safety and efficacy of Mercazole in neonates and very young infants have not been well established, and use in this age group is reserved for rare, specific circumstances (such as neonatal Graves' disease) under close specialist care.