Synthroid vs Plan B: side-by-side comparison
Synthroid (Levothyroxine) and Plan B (Levonorgestrel) both belong to the Hormones and Birth Control class. They share clinical context but use different active ingredients. The choice between them depends on mechanism nuances, side-effect profile and individual response.
| Property | Synthroid | Plan B |
|---|---|---|
| Active ingredient | Levothyroxine | Levonorgestrel |
| Manufacturer | AbbVie | Foundation Consumer Healthcare |
| Class | Hormones and Birth Control | Hormones and Birth Control |
| Strengths | 25mcg, 50mcg, 75mcg, 100mcg, 150mcg | 1.5mg |
| Forms | tablet | tablet |
What's the same
Synthroid and Plan B both belong to the Hormones and Birth Control class and are used for partially overlapping indications. The active ingredients — Levothyroxine vs Levonorgestrel — share the same therapeutic approach, so many safety and management points carry across both.
Key differences
Meaningful differences are in active ingredient (Levothyroxine vs Levonorgestrel), strengths (25mcg, 50mcg, 75mcg, 100mcg, 150mcg vs 1.5mg), forms (tablet vs tablet), and the mechanism, half-life and side-effect nuances that distinguish members of the class.
Mechanism and action
Synthroid: Levothyroxine replaces deficient endogenous thyroxine, which is converted in tissues to the active hormone triiodothyronine (T3) by deiodinase enzymes. Plan B: Plan B's primary mechanism is delaying or preventing ovulation when taken before the LH surge.
When Synthroid is preferred
Synthroid is approved for hypothyroidism of any cause (Hashimoto thyroiditis, post-thyroidectomy, post-radioiodine, congenital), goitre and TSH suppression after differentiated thyroid cancer.
When Plan B is preferred
Plan B is indicated for emergency contraception after unprotected intercourse or known/suspected contraceptive failure, in women of reproductive age.
Frequently asked questions
Is Synthroid or Plan B better? ▾
There is no single answer. Synthroid and Plan B both belong to the Hormones and Birth Control class but differ in mechanism nuances, half-life and side-effect profile. Preference depends on the patient, the prescriber and prior response to other therapies.
Can I switch from Synthroid to Plan B? ▾
Switching within the Hormones and Birth Control class is done under supervision, typically using equivalent doses and a follow-up period to confirm response and tolerance. It is not a self-directed decision.
Do Synthroid and Plan B have the same side effects? ▾
They share many of the Hormones and Birth Control class side effects, with differences from mechanism and dose. Each medication's prescribing information lists specifics.
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The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.