Synthroid®
Levothyroxine
Version 2025-04 · Last reviewed April 1, 2025 · Methodology
List Price
$35
With Insurance
$4
Narrow Therapeutic Index (NTI) Drug
Narrow Therapeutic Index (NTI): Small changes in dose or brand can cause over- or under-treatment. The FDA considers brand and generic levothyroxine therapeutically equivalent, but some endocrinologists recommend staying on the same manufacturer's product once stable. If your pharmacy switches brands, watch for signs of over-replacement (palpitations, anxiety, sweating, weight loss) or under-replacement (fatigue, cold intolerance, weight gain, brain fog) and notify your prescriber. TSH should be rechecked 6–8 weeks after any formulation change.
Supplement Questions
Whey Protein Concentrate
High-protein meals affect levothyroxine absorption timing. Take thyroid medication 30–60 min before protein shakes or food.
Whey Protein Isolate
Protein meals affect levothyroxine absorption timing — take thyroid medication at least 30–60 min before protein supplement or food.
Whey Protein Hydrolysate
Even hydrolyzed protein affects levothyroxine absorption timing — take thyroid medication before protein supplementation.
Casein Protein
Pre-sleep casein protein timing: if taking levothyroxine at night, it should be taken at least 1–2 hours before the casein supplement.
Egg White Protein
Protein meals affect levothyroxine absorption timing. Take thyroid medication at least 30–60 min before protein supplement or food.
Calcium
Calcium is one of the most clinically significant interactions with levothyroxine. A 4-hour separation is required — calcium significantly reduces thyroid hormone absorption.
How It Works
Levothyroxine is synthetic T4 — an exact replica of the thyroid hormone the body can't produce enough of. After absorption, it is converted to the active form (T3) in peripheral tissues, where it regulates virtually every metabolic process in the body.
Why the side effects happen
Levothyroxine side effects are almost entirely dose-related — too much T4 produces hyperthyroid symptoms (palpitations, heat intolerance, anxiety, weight loss, bone loss). The goal is to replace exactly what's missing. Absorption varies substantially based on timing relative to food, coffee, calcium, and iron supplements — all of which can dramatically reduce bioavailability.
When Will I Feel It?
TSH starts normalizing within 2–4 weeks. Full TSH stability takes 6–8 weeks — which is when labs should be rechecked. Symptom improvement takes 2–3 months.
TSH begins responding. Most patients don't notice much change in symptoms yet.
TSH fully stable at new dose level. This is when labs should be checked to confirm dosing.
Full symptom improvement — energy, cold intolerance, weight, brain fog, mood all improving.
Annual TSH monitoring. Dose often needs adjustment after major weight changes, pregnancy, or aging.
Adherence Note
Take on an empty stomach, 30–60 minutes before any food or coffee. Calcium, iron supplements, and antacids must be separated by at least 4 hours — they dramatically reduce absorption. Consistency of timing is more important than the exact time of day.
Medical Disclaimer
The information on this page is compiled from publicly available clinical trial data, FDA prescribing information, and peer-reviewed literature. It is provided for educational purposes only and does not constitute medical advice, diagnosis, or treatment recommendations. Individual responses to medications vary. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.
Common Side Effects
While taking this medication, you may experience the following common side effects. We've included tips on how to manage them.
Heart palpitations
8%Usually means dose is too high; contact doctor immediately
Insomnia / sleep trouble
7%Take in the morning; dose may need adjustment
Anxiety / nervousness
7%Often a sign of over-replacement; TSH test needed
Tremors / shakiness
5%Usually indicates dose too high; report to doctor
Excessive sweating
5%Often sign of over-replacement; check TSH
Weight loss (unintended)
5%If losing weight unintentionally, dose may be too high
Headache
4%Usually mild; monitor and report if persistent
Diarrhea
4%Can be sign of over-replacement
Hair loss (temporary)
3%Often occurs in first 3-6 months; usually resolves
Increased appetite
3%Usually due to improved metabolism; may normalize
Serious Adverse Effects
- • Atrial fibrillation
- • Bone loss (osteoporosis with long-term over-replacement)
- • Angina / heart problems
- • Adrenal crisis (if adrenal insufficiency not addressed)
Drug Interactions
Major Interactions (Avoid)
Moderate Interactions (Caution)
Food Interactions
When to Contact Your Doctor
This medication requires ongoing medical supervision. The following situations warrant a prompt conversation with your prescribing physician — do not wait for your next scheduled appointment.
Contact soon if you notice
- Atrial fibrillation
- Bone loss (osteoporosis with long-term over-replacement)
- Angina / heart problems
- Adrenal crisis (if adrenal insufficiency not addressed)
- Extreme fatigue and brain fog
Also discuss if you want to
- Review whether this medication is still appropriate for you
- Consider dosage adjustments based on response
- Explore lifestyle or non-drug alternatives
- Understand stopping or tapering options
- Plan monitoring labs and follow-up
In the US, call 911 or go to the nearest emergency room for severe symptoms. Poison Control: 1-800-222-1222.
Special Populations
Safety classifications for specific groups — discuss with your provider before use.
Dose often increases in pregnancy; critical for fetal development.
Natural hormone present in milk; safe for infant.
Hypothyroidism and menopause share many of the same symptoms: fatigue, weight gain, mood changes, brain fog, hair thinning, and sleep problems. Thyroid disease also becomes more common after menopause. Make sure thorough thyroid testing (TSH, free T3, free T4) has been done before attributing all symptoms to menopause — and vice versa. The two conditions frequently occur together.
Critical for normal development; careful monitoring needed.
Start with 25mcg; cardiac risk higher; TSH target 4-6.
FDA Adverse Event Reports
Patient-filed reports from the FDA FAERS database · refreshed daily
Community Reports
User-reported experiences — anonymous & anecdotal
Medical Disclaimer
The information on this page is compiled from publicly available clinical trial data, FDA prescribing information, and peer-reviewed literature. It is provided for educational purposes only and does not constitute medical advice, diagnosis, or treatment recommendations. Individual responses to medications vary. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.
Metabolic & Lifestyle Alternatives
Nutritional & Lifestyle Approaches for Thyroid Health
Certain nutritional deficiencies and autoimmune triggers may be modifiable with dietary and lifestyle changes in some patients
Important context: Evidence quality varies across these approaches. Some are well-studied with randomized controlled trial data; others are based on observational or smaller studies. These interventions are not guaranteed to replace medication for all patients. Discuss with your doctor whether any of these are appropriate for your clinical situation.
Selenium supplementation
200mcg/day
Reduces thyroid antibodies in Hashimoto's by 50%
Gluten-free diet (Hashimoto's)
Remove gluten entirely
Reduces antibodies, may improve TSH
Vitamin D
2000-4000 IU/day
Deficiency linked to autoimmune thyroid disease
Iodine (moderate)
Adequate not excessive
Both deficiency and excess worsen thyroid
Global Prescribing & Pricing
US levothyroxine prescribing rates are notably high, particularly for subclinical hypothyroidism where evidence for treatment benefit in most patients is limited
United States
$10–15 (generic)/mo
Most prescribed medication overall — subclinical treatment is controversial
TSH threshold for treatment varies widely by physician; no standard lifestyle prerequisite
Usually covered
United Kingdom
~$1–6/mo
Comparable prescribing rate; watchful waiting preferred for subclinical cases
NICE recommends observation-only for mild subclinical hypothyroidism; emphasizes iodine sufficiency
Fully covered by NHS
Germany
~$5–11/mo
Slightly lower rate due to iodine fortification programs
Mandatory iodine fortification of table salt reduces hypothyroidism burden; T3/T4 combination available
Covered by GKV
France
~$3–9/mo
Similar rate; dietary causes explored first
National iodine deficiency monitoring program; dietary root causes assessed before prescribing
Covered by Sécurité Sociale
Japan
~$7–20/mo
Lower rate — dietary iodine from seaweed reduces deficiency
High seaweed consumption provides natural iodine; autoimmune monitoring standard; iodine counseling given
Covered by JHIS
Germany and Denmark mandate iodine fortification of table salt — directly reducing the national burden of hypothyroidism and need for levothyroxine. Japan achieves the same through dietary seaweed consumption. The US relies on voluntary salt fortification, resulting in higher treatment rates.
Clinical Trials & Funding
Understanding who funds research helps contextualize results. Industry-funded trials are not automatically invalid — they undergo the same FDA review — but declared conflicts and sponsor effects are worth knowing. All linked trials can be verified on ClinicalTrials.gov.
Funding Sources
AbbVie spent $54M lobbying to prevent generic switching. Endocrinology societies receive millions from pharmaceutical companies. Subclinical treatment pushed despite limited evidence.
Declared Conflicts of Interest
American Thyroid Association receives pharmaceutical sponsorship. Guidelines committees: majority of members have industry ties.
Key Efficacy Results
TSH normalizes in 90%, but symptoms may not resolve
Referenced Studies
Each study carries a Cochrane RoB-2 risk-of-bias badge — tap the badge for details.
Evidence & Transparency
Cochrane RoB-2 (Risk of Bias)
Badges reflect an editorial assessment using Cochrane's RoB-2 tool domains: randomization, intervention deviation, missing data, outcome measurement, and selective reporting. These are not certified Cochrane reviews. Learn more ↗
CMS Open Payments
Manufacturer payment disclosures are reported via the CMS Sunshine Act. Disclosure is legally required and does not imply bias or misconduct. Language uses "may," "suggests," or "appears" — never definitive clinical claims. CMS Open Payments ↗
Live Clinical Trials
Live from ClinicalTrials.gov · refreshed every 4 hours
Currently enrolling, active, and recently completed studies involving Levothyroxine. Data is pulled directly from the U.S. National Library of Medicine.
Recent Research
Live from PubMed · peer-reviewed literature · refreshed every 4 hours
Most recently indexed clinical trials and systematic reviews mentioning Levothyroxine in PubMed.
Source Documentation
Structured citations for referenced clinical trials
Each referenced trial is listed with its registry ID, funding source, and bias assessment. Use the copy button to generate a formatted citation.
| Trial | Registry ID | Cite |
|---|---|---|
| Levothyroxine Bioequivalence | NCT01660126 |
Bias ratings use Cochrane RoB-2 methodology. Editorial assessment — not a certified Cochrane review.
Our MethodologyMedical Disclaimer
The information on this page is compiled from publicly available clinical trial data, FDA prescribing information, and peer-reviewed literature. It is provided for educational purposes only and does not constitute medical advice, diagnosis, or treatment recommendations. Individual responses to medications vary. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.
Stopping This Medication Safely
Stopping causes return of hypothyroid symptoms. Most patients with Hashimoto's or post-thyroidectomy require lifelong therapy. Only consider stopping with confirmed remission and endocrinologist guidance.
What Published Research Shows About Stopping This Medication
This summarizes what published research documents — it is not personal medical advice. Any changes to your medication require discussion with your prescribing physician.
- ·Research consistently supports attempting this process only with endocrinologist supervision and documented remission criteria
- ·Published protocols describe dose reduction of 25mcg every 6 weeks with TSH monitoring at each step
- ·Some research supports selenium 200mcg, adequate vitamin D, and appropriate iodine intake to support thyroid function during this process
- ·Research indicates that TSH normalization achieved only through medication (not thyroid recovery) suggests ongoing treatment may be needed
Warning Symptoms — Contact Your Doctor If You Experience:
- Extreme fatigue and brain fog
- Cold intolerance
- Weight gain (unexplained)
- Hair loss or brittle nails
- TSH rising above 4.5 mIU/L
Never change or stop a medication without consulting your prescribing physician.
Questions for Your Doctor
Questions to Ask
- 1.Is this Hashimoto's autoimmune thyroiditis?
- 2.Should we check T3 as well as TSH?
- 3.Would diet changes help my thyroid antibodies?
- 4.Is my dose the right strength?
Lab Tests to Request
- TSH
- Free T4
- Free T3
- Thyroid antibodies (TPO, TG)
- Vitamin D
- Selenium
Medical Disclaimer
The information on this page is compiled from publicly available clinical trial data, FDA prescribing information, and peer-reviewed literature. It is provided for educational purposes only and does not constitute medical advice, diagnosis, or treatment recommendations. Individual responses to medications vary. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.
Frequently Asked Questions About Synthroid®
- What is Synthroid® used for?
- Synthroid® (Levothyroxine) is a Thyroid Hormone manufactured by AbbVie. FDA-approved indications include: Hypothyroidism; Thyroid cancer; Goiter.
- What are the common side effects of Synthroid®?
- Common side effects of Synthroid® include: Heart palpitations (8%); Insomnia / sleep trouble (7%); Anxiety / nervousness (7%); Tremors / shakiness (5%); Excessive sweating (5%).
- How much does Synthroid® cost?
- Synthroid® list price is approximately $35. With insurance it typically costs $4; without insurance approximately $10-30.
- Who funded the clinical trials for Synthroid®?
- AbbVie spent $54M lobbying to prevent generic switching. Endocrinology societies receive millions from pharmaceutical companies. Subclinical treatment pushed despite limited evidence.
- How strong is the clinical evidence for Synthroid®?
- Key studies: Levothyroxine's approval predated modern trials; most recent trials on subclinical hypothyroidism. TSH normalizes in 90%, but symptoms may not resolve Potential conflicts of interest: American Thyroid Association receives pharmaceutical sponsorship. Guidelines committees: majority of members have industry ties..
- Are there non-drug alternatives to Synthroid®?
- Certain nutritional deficiencies and autoimmune triggers may be modifiable with dietary and lifestyle changes in some patients See the Alternatives tab for full details.
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