Levothyroxine (T4, Synthroid)
Synthetic T4 (3,5,3',5'-tetraiodothyronine), Knoll/AbbVie 1955 FDA-approved hypothyroidism Rx gold standard (Synthroid). A prohormone – hepatic and renal 5'-deiodinase (D1/D2) convert it to active T3. Off-label cutting-cycle use is less common than T3 (slower 7-day t1/2, steady-state ~6 weeks). WADA: allowed with documented hypothyroidism diagnosis.
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WHAT IS LEVOTHYROXINE (T4, SYNTHROID)?
Detailed overview
Levothyroxine (Synthroid) is the synthetic form of endogenous T4 (3,5,3',5'-tetraiodothyronine), the prohormone form of thyroid hormone. It received FDA approval in 1955 under the Knoll Pharmaceutical (now AbbVie) Synthroid brand and, with 70 years of clinical literature, has been a US top-5 prescribed drug for the past two decades. As a hypothyroidism Rx gold standard per the Garber 2012 AACE/ATA guideline (PMID 22293194) and Jonklaas 2014 ATA guideline (PMID 25266247): TSH target is 0.5-2.5 mIU/L, Rx dose 1.6 μg/kg/day (60-100 kg → 100-150 μg/day). In peripheral tissues (liver, kidney, muscle, central nervous system), T4 is converted to active T3 by the 5'-deiodinase D1/D2 enzymes (~80% peripheral T4 → T3 conversion), and only indirectly acts as a nuclear TR-α/β agonist because T4 itself has low receptor affinity (~10x weaker than T3). Off-label cutting-cycle use is less common than with T3: the slower on/off kinetics (7-day t1/2 vs T3's 24 hours) give a ~6-week steady state and a slower metabolic-rate increase. Stott 2017 TRUST NEJM (PMID 28121505), an older-adult subclinical-hypothyroidism RCT, found no symptomatic benefit from T4 substitution, yet the prescribing rate did not decline in the US. On the WADA list, T4 is in the 'allowed' category with documented hypothyroidism diagnosis – athletic cutting-cycle off-label use is NOT legitimate and constitutes a potential WADA violation.
Mechanism of action
Thyroid prohormone, peripheral D1/D2 conversion to T3, indirect nuclear TR-α/β agonist
Dosing
Clinical 1.6 μg/kg/day (100-150 μg/day); off-label cutting 200-300 μg/day (NOT recommended)
Half-life
~7 days (steady-state ~6 weeks)
Onset
Metabolic-rate increase 1-2 weeks; steady-state 6 weeks
Legal status
FDA + EMA Rx (hypothyroidism), registered in HU + PL. WADA: allowed with documented hypothyroidism diagnosis, off-label cutting is NOT legitimate.
Data console
Lab data
Safety
Side effects, stop signs, contraindications
Side effects · 8
- Iatrogenic hyperthyroidism on overdose: tachycardia, palpitations, atrial fibrillation, heat intolerance, sweating, weight loss, tremor (FDA Synthroid SmPC; most pronounced on supratherapeutic cutting doses).
- Atrial fibrillation and cardiovascular event risk, especially in the elderly and those with pre-existing heart disease; suppressive (TSH <0.1) dosing raises AF incidence 2–3x (Sawin 1994 PMID 8302057).
- Decreased bone density and osteoporosis risk on long-term TSH-suppressive dosing (>200 µg/day, chronic TSH <0.1 mIU/L); DEXA monitoring warranted (Faber 1994 PMID 8112375).
- Central/nervous overdose symptoms: anxiety, irritability, insomnia, restlessness, headache (part of the hyperthyroid syndrome).
- Reduced absorption and dose instability when taken with calcium, iron, PPIs or soy (~30–50% absorption drop); 4-hour separation required.
- Warfarin interaction: T4 potentiates the anticoagulant effect, raising INR and bleeding risk; INR monitoring mandatory.
- In diabetics, the increased metabolic rate can raise insulin and antidiabetic requirements, with possible loss of glucose control.
- Rare cholestasis and <1% ALT elevation; hepatic stress is minimal, though overdose-induced hyperthyroidism is indirectly burdensome (FDA Synthroid SmPC).
Contraindications · 7
- Uncorrected adrenal insufficiency: T4 replacement can precipitate an adrenal crisis; glucocorticoid replacement must come first (absolute contraindication).
- Untreated thyrotoxicosis / hyperthyroidism: T4 further aggravates the thyroid hormone excess (absolute contraindication).
- Recent acute myocardial infarction (<6 months): the increased cardiac workload contraindicates dose titration (absolute contraindication).
- Active tachyarrhythmia (atrial fibrillation, ventricular tachycardia): T4 is arrhythmogenic; cautious titration or deferral is warranted.
- Athlete cutting-cycle off-label use without a documented hypothyroidism diagnosis: medically illegitimate and a potential WADA violation.
- Elderly (>65) and coronary artery disease patients: relative caution, low starting dose and slow (12.5 µg/2 weeks) titration warranted (Stott 2017 TRUST PMID 28121505).
- Chronic TSH-suppressive dosing (>200 µg/day) with osteoporosis or known low bone density: relative contraindication due to bone density loss.
Related Performance Compounds
Same therapeutic category
Studies
Related research and clinical findings
Discontinuation of levothyroxine therapy in patients with subclinical hypothyroidism: a pilot randomized clinical trial.
Maraka S, Owen RR, Singh Ospina NM
A 2013 survey of clinical practice patterns in the management of primary hypothyroidism.
Burch HB, Burman KD, Cooper DS
Guidelines for the treatment of hypothyroidism (ATA)
Jonklaas J, Bianco AC, Bauer AJ et al.
Combined levothyroxine plus liothyronine compared with levothyroxine alone in primary hypothyroidism: a randomized controlled trial
Walsh JP, Shiels L, Lim EM et al.
Levothyroxine Formulations: Pharmacological and Clinical Implications of Generic Substitution.
Benvenga S, Carlé A
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The information here is for educational and scientific purposes only. Performance-enhancing compounds (AAS, prohormones, stimulants, doping agents) are illegal without prescription in Hungary and most of the EU, and carry serious health and legal risks. WADA bans them in competitive sport. This is NOT a usage guide, and we do not encourage any illegal use. If you do use them, medical supervision and regular bloodwork are ESSENTIAL. Severe endocrine, cardiovascular, hepatic and psychiatric side effects are possible.