Is Synthroid (levothyroxine) used to treat hypothyroidism?

Synthroid (Levothyroxine) for Hypothyroidism

Yes, Synthroid (levothyroxine) is the first-line treatment for hypothyroidism and is indicated as replacement therapy in primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) congenital or acquired hypothyroidism. 1

Indications for Levothyroxine Therapy

• Levothyroxine is the principal treatment for hypothyroidism, with synthetic levothyroxine (such as Synthroid) considered the drug of choice due to its predictable and reliable effects 2
• It is indicated for replacement therapy in all forms of hypothyroidism (primary, secondary, and tertiary) 1
• Levothyroxine is also used as an adjunct to surgery and radioiodine therapy in managing thyrotropin-dependent well-differentiated thyroid cancer 1

Diagnosis and Assessment Before Treatment

• Diagnosis of hypothyroidism is based on biochemical testing: elevated TSH with low free T4 indicates overt primary hypothyroidism 3
• Abnormal TSH findings should be confirmed with repeat testing over a 3-6 month interval, as 30-60% of high TSH levels normalize on repeat testing 4
• Multiple tests should be performed to establish persistence of the abnormality before initiating treatment 5

Treatment Algorithm Based on TSH Levels

• For patients with TSH >10 mIU/L (regardless of symptoms) or any TSH elevation with symptoms, levothyroxine therapy is recommended 4
• For subclinical hypothyroidism with TSH between 4.5-10 mIU/L and normal T4, treatment decisions should be individualized based on symptoms, presence of TPO antibodies, and risk factors 4
• The median TSH level at which levothyroxine therapy is typically initiated has decreased from 8.7 to 7.9 mIU/L in recent years 4

Dosing Guidelines

• Initial dosage depends on age, body weight, cardiovascular status, and comorbid conditions 1
• For patients <70 years without cardiac disease, the full replacement dose is approximately 1.6 mcg/kg/day 4
• For patients >70 years or with cardiac disease, start with a lower dose of 25-50 mcg/day and titrate gradually 4
• Administration should be as a single daily dose, on an empty stomach, 30-60 minutes before breakfast with a full glass of water 1

Monitoring and Dose Adjustments

• Monitor TSH every 6-8 weeks while titrating hormone replacement 4
• Once adequately treated, repeat testing every 6-12 months or if symptoms change 4
• For primary hypothyroidism, titrate until the patient is clinically euthyroid and serum TSH returns to normal 1
• For secondary or tertiary hypothyroidism, use serum free-T4 levels (not TSH) to guide therapy 1

Clinical Benefits of Treatment

• Treatment of hypothyroidism with levothyroxine improves symptoms including fatigue, weight gain, cognitive issues, and menstrual irregularities 3
• Proper treatment reduces the risk of serious complications such as heart failure and myxedema coma 3
• Levothyroxine therapy can improve insulin resistance in patients with diabetes and reduce cardiovascular risks 3

Common Pitfalls and Considerations

• Avoid using thyroid hormones for weight loss or obesity treatment, as this is not an approved indication and can cause serious toxicity 1
• About 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, highlighting the importance of regular monitoring 4
• Overtreatment with levothyroxine can lead to iatrogenic hyperthyroidism, increasing risk for osteoporosis, fractures, abnormal cardiac output, and ventricular hypertrophy 4
• Levothyroxine should be taken at least 4 hours before or after drugs known to interfere with its absorption 1

Special Populations

• For pregnant women or those planning pregnancy, more aggressive normalization of TSH is warranted, as subclinical hypothyroidism during pregnancy is associated with adverse outcomes 4
• In elderly patients, treatment should be started with lower doses and titrated gradually to avoid cardiac complications 5

Synthroid and generic levothyroxine appear to have similar efficacy in treating hypothyroidism, with one study in children with congenital hypothyroidism showing similar or even better control with generic levothyroxine compared to Synthroid 6.