Generic Name

brand name formulation of levothyroxine sodium

Brand Names

formulation of levothyroxine sodium**, a synthetic form of the thyroid hormone thyroxine (T4). It is widely used to treat hypothyroidism and certain thyroid disorders in both clinical and research settings.

Mechanism

The drug Unithroid exerts its effects through the replacement or supplementation of endogenous thyroid hormone.
• Cellular uptake: T4 is absorbed in the gastrointestinal tract and released into the bloodstream; it then diffuses into peripheral tissues.
• Peripheral conversion: In most target tissues, 5‑deiodinase enzymes convert T4 to the biologically active hormone triiodothyronine (T3).
• Receptor binding: T3 binds to nuclear thyroid hormone receptors (TRα and TRβ), altering transcription of numerous genes that regulate metabolism, development, and thermogenesis.
• Feedback inhibition: Elevated circulating T4/T3 suppresses thyrotropin‑releasing hormone (TRH) and thyroid‑stimulating hormone (TSH) secretion via negative feedback on the hypothalamic‑pituitary‑thyroid axis.

Pharmacokinetics

• Absorption: Peak plasma conc. after oral dose reached within 2–6 h; bioavailability ~ 70–80 % under fasting conditions.
• Food effect: Consuming food impairs absorption by up to 25 %; best taken 30–60 min before breakfast.
• Distribution: Highly protein‑bound (> 99 % to thyroxine‑binding globulin, transthyretin, albumin).
• Metabolism: Primarily hepatic deiodination to T3 and reverse T3.
• Elimination: 90 % excreted in feces, 10 % in urine; half‑life 6–7 days (T4) and 1–2 days (T3).
• Special populations:
• *Pediatric*: Dose adjustment based on weight; half‑life ~6 days.
• *Renal impairment*: No dose modification needed (minimal renal excretion).
• *Pregnancy*: Gestation increases TBG, requiring dose increase (~30 % by 3rd trimester).

Indications

The drug Unithroid is indicated for:
• Primary hypothyroidism (e.g., Hashimoto’s thyroiditis, post‑thyroidectomy).
• Secondary hypothyroidism (pituitary failure) when combined with TSH support.
• Thyroid hormone replacement in neonatal and pediatric hypothyroidism.
• Certain endocrine disorders requiring controlled thyroid hormone levels (e.g., euthyroid sick syndrome).

Contraindications

• Contraindicated in:
• Acute thyrotoxic crisis (e.g., thyroid storm).
• Uncontrolled cardiovascular disease (unstable angina, recent MI).
• Active peptic ulcer disease.
• Warnings:
• Cardiovascular: May precipitate arrhythmias or ischemia in susceptible patients.
• Pituitary disorders: May worsen hyperthyroidism if undetected.
• Adrenal insufficiency: Requires stress‑dose glucocorticoid prior to initiating therapy.
• Drug interactions:
• Calcium supplements, iron, antacids, cholestyramine, warfarin, oral contraceptives can reduce absorption.
• Beta‑blockers, diazoxide, high‑dose steroid therapy can alter efficacy.

Dosing

• Adults:
• Start: 0.05–0.1 µg/kg/day (typically 50–75 µg).
• Titrate upward every 4–6 weeks based on TSH and free T4 levels.
• Target TSH: 0.5–4.5 mIU/L (or institution‑specific range).
• Pediatric:
• 1–1.5 µg/kg/day, adjust to maintain age‑appropriate free T4.
• Route: Oral, 30–60 min before breakfast, on an empty stomach.
• Administration tips:
• Wash the pill with water; do not chew or crush.
• Keep refrigerated with tight seal; avoid temperatures > 25 °C.

Adverse Effects

Unithroid is generally well tolerated, but common side effects include:
• Mild: nausea, headache, irritability, mild tachycardia.
• Serious:
• *Tachyarrhythmias* (atrial fibrillation, supraventricular tachycardia).
• *Osteoporosis* with prolonged high dosing.
• *Thyroid‑induced myopathy* (rare).
• *Allergic reactions* (rash, anaphylaxis).

The incidence of serious adverse events is low (< 1 %) when therapeutic windows are respected.

Monitoring

Clinical Pearls

• Start low, go slow: Even a modest dose change can dramatically alter TSH; titrate cautiously.
• Timing is everything: A 30‑minute pre‑breakfast window dramatically improves bioavailability—critical for students who mix doses with breakfast.
• Switching brands without counseling: If a patient moves from a generic to Unithroid, verify equivalence to avoid inadvertent overdosing.
• Titrate in the dark: Use a blinded TSH assay result to guide dose changes; avoid subjective symptom‑only adjustments.
• Interacting drugs: Remember that over-the-counter calcium and antacid tablets diminish absorption; advise patients to separate intake by ≥ 4 h.
• Pregnancy: Because Unithroid crosses the placenta, dose up 30 % by the third trimester and monitor closely to avoid fetal hyperthyroidism or fetal growth restriction.
• Pituitary testing: Prior to initiating therapy in patients with suspected pituitary disease, obtain baseline ACTH and cortisol to safeguard against precipitating adrenal crisis.

*These pearls are intended to enhance clinical decision‑making for medical students, residents, and practicing clinicians. Always refer to institutional protocols and the latest clinical guidelines for definitive treatment plans.*