左甲狀腺素（Levothyroxine）
Overview
Levothyroxine is a synthetic form of thyroxine (T4), an endogenous hormone secreted by the thyroid gland. It is primarily used as a replacement therapy for hypothyroidism and to suppress thyroid-stimulating hormone (TSH) in the management of thyroid nodules and cancer.
Mechanism of Action
It acts as a replacement for endogenous thyroid hormone. Once in the body, it is converted to its active metabolite, triiodothyronine (T3). T3 enters cells and binds to thyroid hormone receptors in the nucleus, regulating gene expression, metabolism, growth, and development.
Dosage and Administration
The dosage is highly individualized based on age, weight, cardiovascular status, and the severity of hypothyroidism. It is typically taken once daily on an empty stomach, at least 30 to 60 minutes before breakfast, to ensure optimal and consistent absorption.
Side Effects
Adverse effects usually result from overdosage, mimicking symptoms of hyperthyroidism. These include palpitations, tachycardia, arrhythmias, weight loss, heat intolerance, sweating, tremors, nervousness, and insomnia. Long-term suppressive therapy may decrease bone mineral density.
Contraindications
It is contraindicated in patients with untreated subclinical or overt thyrotoxicosis, acute myocardial infarction, and uncorrected adrenal insufficiency. Caution is advised in patients with cardiovascular disorders and the elderly.
Pregnancy and Lactation
Levothyroxine is safe and necessary during pregnancy for women with hypothyroidism, as maternal thyroid hormone is crucial for fetal brain development. Dose requirements typically increase during pregnancy. It is also safe during lactation.
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