Unithroid - Drug Monograph

Introduction

Unithroid (levothyroxine sodium) is a synthetic thyroid hormone preparation used as replacement therapy in hypothyroidism. It is identical to the endogenous hormone produced by the human thyroid gland and represents the standard of care for thyroid hormone replacement therapy. Unithroid is available in tablet form and is classified as a thyroid agent.

Mechanism of Action

Levothyroxine sodium is a synthetic form of thyroxine (T4), which undergoes peripheral deiodination to triiodothyronine (T3), the more active thyroid hormone. Thyroid hormones exert their effects by binding to thyroid hormone receptors that regulate gene transcription, ultimately influencing metabolic processes throughout the body. They increase metabolic rate, enhance carbohydrate, protein, and fat metabolism, stimulate protein synthesis, and are essential for normal growth and development, particularly of the central nervous system.

Indications

• Treatment of hypothyroidism
• TSH suppression in the management of thyroid cancer (adjunct therapy)
• Thyrotropin-dependent hypothyroidism (pituitary or hypothalamic origin)
• Myxedema coma (as part of combination therapy)
• Diagnostic agent in thyroid suppression tests

Dosage and Administration

• Adults: 25-50 mcg daily, increased by 25-50 mcg increments every 2-3 weeks
• Elderly or cardiac patients: Start with 12.5-25 mcg daily
• Children: Based on body weight and age (dosing varies significantly by developmental stage)

• Take on an empty stomach, 30-60 minutes before breakfast
• Tablets should be swallowed whole with water
• Consistent daily timing is essential for stable hormone levels
• Do not crush or chew tablets

• Pregnancy: Dose requirements may increase by 25-50%
• Elderly: Lower initial doses recommended
• Renal impairment: No dosage adjustment typically needed
• Hepatic impairment: Use with caution; monitor closely

Pharmacokinetics

Contraindications

• Untreated subclinical or overt thyrotoxicosis
• Acute myocardial infarction
• Uncorrected adrenal insufficiency
• Hypersensitivity to any component of the formulation
• Un-treated pituitary or hypothalamic disorders (unless properly diagnosed and managed)

Warnings and Precautions

• Cardiac effects: May precipitate angina pectoris, arrhythmias, or myocardial infarction in patients with cardiovascular disease
• Adrenal insufficiency: May precipitate adrenal crisis in patients with uncontrolled adrenal insufficiency
• Osteoporosis: Long-term supraphysiologic doses may decrease bone mineral density
• Elderly patients: Increased risk of atrial fibrillation and other cardiac complications
• Pregnancy: Requires careful monitoring and possible dose adjustments
• Diabetes mellitus: May alter glycemic control; monitor blood glucose closely

Drug Interactions

• Antacids, calcium carbonate, iron supplements: Decrease absorption (separate by at least 4 hours)
• Bile acid sequestrants (cholestyramine, colestipol): Decrease absorption
• Estrogens: May increase thyroid-binding globulin, requiring dose adjustment
• Warfarin: Potentiates anticoagulant effect (monitor INR closely)
• Antidiabetic agents: May alter glycemic control
• Ketamine: May precipitate hypertension and tachycardia
• Tricyclic antidepressants: Mutual potentiation of effects
• Digoxin: May decrease serum digoxin levels
• Phenytoin, carbamazepine: Increase metabolism of levothyroxine

Adverse Effects

• Palpitations
• Tachycardia
• Increased appetite
• Weight loss
• Nervousness
• Insomnia
• Tremor
• Headache
• Hyperhidrosis

• Cardiac arrhythmias
• Angina pectoris
• Myocardial infarction
• Heart failure
• Thyroid storm (with overdose)
• Adrenal crisis (in susceptible patients)
• Severe allergic reactions (rare)

Monitoring Parameters

• Clinical assessment: Symptoms of hypothyroidism/hyperthyroidism, cardiac status
• Thyroid function tests: TSH (primary monitoring parameter), free T4
• Cardiac monitoring: Heart rate, blood pressure, EKG in patients with cardiac history
• Bone density: In patients on long-term supraphysiologic doses
• Growth and development: In pediatric patients
• Pregnancy: More frequent monitoring (every 4-6 weeks)

Patient Education

• Take medication consistently at the same time each day, on an empty stomach
• Do not switch between different levothyroxine products without medical supervision
• Separate administration from calcium supplements, iron, antacids, and certain foods by at least 4 hours
• Report symptoms of hyperthyroidism (palpitations, sweating, nervousness, weight loss) or persistent hypothyroidism
• Do not discontinue medication without consulting healthcare provider
• Inform all healthcare providers about thyroid medication use
• Keep regular follow-up appointments for monitoring
• Store medication at room temperature, away from moisture and light

References

1. FDA Prescribing Information: Unithroid (levothyroxine sodium) tablets 2. American Thyroid Association Guidelines for the Treatment of Hypothyroidism. Thyroid. 2014;24(12):1670-1751 3. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670-1751 4. Drug Facts and Comparisons. Facts & Comparisons [database online]. St. Louis, MO: Wolters Kluwer Health Inc; 2023 5. Lexicomp Online, Hudson, Ohio: Wolters Kluwer Clinical Drug Information, Inc.; 2023 6. Surks MI, Ortiz E, Daniels GH, et al. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. JAMA. 2004;291(2):228-238 7. Wiersinga WM, Duntas L, Fadeyev V, et al. 2012 ETA Guidelines: The Use of L-T4 + L-T3 in the Treatment of Hypothyroidism. Eur Thyroid J. 2012;1(2):55-71