Levothyroxine Sodium (T4)

Pharmacokinetics

Cellular Metabolism and Receptor Activation

1. Conversion of T4 to T3 by Peripheral Deiodinases: Levothyroxine is actively taken up by cells through transporters MCT8 and OATP1C1. In the cellular cytosol, the enzymes deiodinases type 1 and type 2 (D1, D2) remove the iodine atom from the outer ring of T4 to produce the metabolically active hormone: Triiodothyronine (T3):

L-T₄ Deiodinase D1 / D2 L-T₃ (Affinity ≈ 10-15 times higher than the receptor)

2. Nuclear Translocation and Binding to the Thyroid Hormone Receptor (TR): T3 penetrates the cell nucleus and binds with high affinity to the thyroid hormone receptor, which forms heterodimers with the RXR receptor and is permanently bound to the thyroid hormone response elements (TREs) of the chromosomal DNA.

3. Induction of Expression of Key Metabolic Genes:

• Na+/K+ ATPase: Increases the synthesis of the sodium-potassium ATPase pump, directly increasing basal oxygen consumption and thermogenesis (calorigenic effect).
• Beta-1 Adrenergic Receptors: Increases the expression of these receptors in the myocardial tissue, increasing heart rate, contractility and cardiac output (positive chronotropic and inotropic effect).
• Sarcoplasmic Reticulum Ca2+ ATPase (SERCA2): Optimizes the relaxation speed of the cardiac muscle in diastole.
• Lipid and Carbohydrate Metabolism: Increases hepatic LDL receptors (decrease circulating cholesterol) and accelerates intestinal absorption of complex carbohydrates.

High Resolution Pharmacokinetics

• Absorption and Bioavailability: Bioavailability between 60% and 80% on an empty stomach. It is mainly absorbed in the jejunum and ileum. Normal gastric acidity is essential to dissolve the levothyroxine tablet. Foods, coffee, tea and certain cations (calcium, iron) substantially reduce intestinal absorption.
• Distribution: Apparent volume of distribution of approximately 11.6 L. Extreme binding to circulating plasma proteins (>99.97%), mainly to Thyroxine Binding Globulin (TBG), Thyroxine Binding Prealbumin (TBPA) and albumin. Only 0.03% circulates in free and metabolically active form (free T4).
• Metabolism: Mainly tissue deiodination in the liver, kidneys and skeletal muscle. Minor hepatic conjugation with glucuronic acids and sulfates eliminated via the bile.
• Excretion: 50% via the kidneys in the form of deiodinated metabolites; 50% for feces.
• Half-life (t_1/2): Very long, 6 to 8 days in euthyroid patients (prolonging to 9-10 days in patients with hypothyroidism and shortening to 3-4 days in frank hyperthyroidism). Allows single daily dosing and forgives occasional forgetfulness.

Indicators and dose

Clinical Indications and Off-Label Uses

• Primary, Secondary or Tertiary Hypothyroidism: Lifelong hormone replacement therapy (Hashimoto's disease, post-surgical hypothyroidism, post-radioactive iodine).
• TSH suppression in Differentiated Thyroid Carcinoma: To prevent the trophic stimulation of TSH on possible foci of metastasis or local recurrence.
• Myxematous Coma: Acute treatment of choice intravenously along with basal glucocorticoid support.

Dosage and Clinical Adjustment

Primary Hypothyroidism in Adults:

• The estimated full replacement dose is approximately 1.6 to 1.8 µg/kg/day orally, preferably calculated based on the patient's lean or ideal weight.
• The tablet should be taken strictly in the morning on an empty stomach, at least 30 to 60 minutes before breakfast, only with a glass of plain water to ensure optimal dissolution.

Special Risk Populations (Adults over 60 years of age or with underlying ischemic heart disease):

• The golden rule of titration is strictly applied: «Start low and go slow».
• Start with very low doses of 12.5 µg to 25 µg once a day, increasing the dose in steps of 12.5-25 µg every 6-8 weeks depending on cardiovascular tolerance and TSH values.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Thyrotoxicosis or untreated acute clinical hyperthyroidism.
• Recent decompensated acute myocardial infarction, acute myocarditis or pancarditis.
• Uncorrected primary adrenal insufficiency (Addison's Disease): The initiation of therapy can trigger an acute Addisonian crisis by accelerating renal and hepatic clearance of the scarce circulating reserve levels of cortisol.

Relative Contraindications

• Stable ischemic heart disease or chronic ventricular arrhythmias (requires ultra-slow dose titration).
• Severe postmenopausal osteoporosis (due to induced risk of accelerated bone resorption in subclinical overdose).

Adverse Effects (ADR) and Specific Toxicity

• ADRs are almost exclusively linked to iatrogenic overdose (inducing a clinical picture of artificial hyperthyroidism / thyrotoxicosis of drug origin).
• Overdose Symptoms:
  • Cardiovascular: Sinus tachycardia, palpitations, de novo atrial fibrillation (especially in older adults), exertional angina pectoris, high output heart failure.
  • Neurological: Extreme nervousness, emotional lability, fine tremor of distal hands, sleep insomnia, headaches.
  • Metabolic: Involuntary weight loss despite increased appetite, diaphoresis, marked intolerance to environmental heat.
  • Osteomuscular: Accelerated loss of bone mass density, proximal muscle fatigue (thyrotoxic myopathy).

Clinical Pearl: Monitoring and Therapeutic Stabilization

Due to the long half-life of levothyroxine (≈ 7 days), the plasma steady state of the hormone takes 6 to 8 weeks to be reached after a dose change or initiation. Therefore, it is a common error in clinical practice to perform serum TSH determinations before this period of metabolic stabilization, except in exceptional situations where severe thyrotoxicosis is suspected.

Drug Interactions of Clinical Relevance

• Absorption chelators (calcium carbonate, ferrous sulfate, aluminum hydroxide, cholestyramine, sucralfate): They markedly decrease the gastric absorption of levothyroxine. They must be physically separated by at least 4 hours from the administration of thyroid hormone.
• Enzyme inducers (Rifampicin, Carbamazepine, Phenytoin): They accelerate deiodination metabolism and hepatic clearance of T4, forcing an increase in the replacement dose of levothyroxine.
• Oral estrogens (contraceptives or hormone replacement therapy): They increase hepatic synthesis of Thyroxine Binding Globulin (TBG). This increases plasma binding sites and temporarily decreases active free T4, increasing overall levothyroxine requirements.

Safety in Pregnancy and Breastfeeding

• Pregnancy: FDA Category A. The early neurological development of the fetus depends exclusively on the transplacental supply of thyroid hormone of maternal origin during the first trimester of pregnancy. Levothyroxine requirements usually increase by 30-50% immediately after conception. The dose of levothyroxine should be adjusted to maintain TSH in the normal range specific to each trimester of pregnancy.
• Breastfeeding: Absolutely compatible. It is excreted in breast milk in minimal concentrations unable to interfere with the infant's thyroid axis or mask congenital neonatal hypothyroidism.