Synthroid (Levothyroxine): A Comprehensive Overview
Synthroid is a brand name for levothyroxine sodium, a synthetic form of thyroxine (T4) that is chemically identical to the hormone naturally produced by the thyroid gland and is the standard treatment for hypothyroidism.
Pharmacology and Mechanism of Action
Levothyroxine works by replacing the missing thyroid hormone in individuals with underactive thyroid (hypothyroidism). It functions through the following mechanisms:
- Controls DNA transcription and protein synthesis by diffusing into cell nuclei and binding to thyroid receptor proteins attached to DNA 1
- Produces physiological actions predominantly through T3 (triiodothyronine), with approximately 80% of T3 derived from the conversion of T4 in peripheral tissues 1
- Maintains normal metabolic functions throughout the body, including regulation of energy, growth, and neurological development 1
Clinical Uses
Levothyroxine is indicated for:
- Primary treatment of hypothyroidism: Used as replacement therapy in primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) hypothyroidism 2, 1
- Treatment of subclinical hypothyroidism: Generally recommended when TSH exceeds 10 mIU/L 2
- Adjunct therapy in thyroid cancer: Used in the management of thyrotropin-dependent well-differentiated thyroid cancer 1
- Prevention of goiter progression: May be used in nontoxic multinodular goiter 3
Dosing and Administration
- Standard starting dose is typically 1.6 mcg/kg body weight per day (usually 75-100 mcg/day for women and 100-150 mcg/day for men) 3
- Available in multiple strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg 1
- Dosage should be adjusted based on TSH levels, with monitoring every 6-12 months for untreated subclinical hypothyroidism and every 2-3 months for treated patients 2
- For elderly patients, initiate at less than the full replacement dose due to increased risk of cardiovascular effects 1
Monitoring and Laboratory Testing
- TSH testing has high sensitivity (98%) and specificity (92%) for confirming thyroid disease 2
- Treatment goal is to normalize serum TSH concentration 2
- Free T4 levels typically remain in the upper half of the normal reference range during appropriate treatment 4
- Laboratory tests should be performed no earlier than 6 weeks after dose adjustments, as this is when a new equilibrium is reached 4
Potential Side Effects and Complications
Overtreatment can lead to:
- Cardiac issues: Atrial arrhythmias, atrial fibrillation (especially in elderly) 1
- Bone mineral density loss 2
- Symptoms of thyrotoxicosis 2
Untreated hypothyroidism can result in:
- Cardiovascular disease and increased mortality 2
- Progression from subclinical to overt hypothyroidism at a rate of 2.6-4.3% per year 2
- In pregnancy: preeclampsia, preterm delivery, heart failure, and miscarriage 2
- Potential contribution to Non-Alcoholic Fatty Liver Disease (NAFLD) 5
Special Considerations
- Pregnancy: Adequate treatment during pregnancy is crucial to prevent adverse obstetric outcomes and fetal cognitive development issues 2
- Pediatrics: Rapid restoration of normal T4 levels is essential in children to prevent adverse effects on cognitive development and physical growth 1
- Elderly: Lower starting doses are recommended due to increased risk of cardiovascular effects 1
- Drug interactions: Many medications can affect T4 absorption and metabolism, requiring dose adjustments 4
Clinical Pearls
- Bioavailability of levothyroxine is approximately 81% when taken properly 6
- Absorption is increased by fasting and decreased by certain foods (particularly soy products) and dietary fiber 1
- For optimal absorption, levothyroxine should be taken consistently either with or without food, preferably on an empty stomach
- Generic and brand name formulations may have different bioequivalence, which can affect dosing requirements 4
Synthroid (levothyroxine) remains the standard of care for hypothyroidism treatment due to its reliable efficacy, once-daily dosing convenience, and well-established safety profile when properly monitored.