T4 (Levothyroxine)
Also known as: Levothyroxine, Synthroid, Euthyrox, L-Thyroxine, Thyroxine, Tetraiodothyronine, T4
Lower numbers = closer to natural. Higher numbers = more enhanced.
Why this rating?
T4 is the storage/prohormone form of thyroid hormone that must be converted to the active T3 in peripheral tissues before it does anything. Because of that conversion step (and the body down-regulating it under negative feedback), T4 is a slower, milder and more self-limiting metabolic tool than T3, roughly 1/4 the cellular potency mcg-for-mcg. It raises metabolic rate and supports fat loss but does not touch anabolic hormones and is NOT on the WADA Prohibited List. Rated 4 as a genuine but gentle metabolic enhancer, one point below T3 which is the aggressive form.
Overview
The main hormone secreted by the thyroid gland and the standard replacement therapy for hypothyroidism. T4 is essentially a prohormone: it is largely inactive until deiodinase enzymes convert it into the far more potent T3 inside peripheral tissues. In a physique context it is used to raise metabolic rate and support fat loss, but it works slower and more gently than T3 and is generally viewed as the milder, more physiological way to run thyroid.
Important Warnings
- •Suppresses the thyroid axis - taper down, do not stop abruptly (rebound hypothyroidism)
- •Cardiac caution: can provoke tachycardia/atrial fibrillation, especially in older users or with stimulants
- •Do not use for fat loss in people with normal thyroid function without understanding the suppression trade-off
- •Take fasted and separate from calcium/iron/magnesium/coffee by 3-4 hours or absorption drops
- •Monitor thyroid function (TSH, free T3, free T4) with bloodwork
- •High doses become catabolic - keep protein high and doses sensible
- •Long-term or high-dose use can reduce bone mineral density - use caution with existing osteoporosis/bone density issues
Purpose & Use Cases
Metabolic Rate Support / Fat Loss
Raises basal metabolic rate and thermogenesis via its conversion to T3, supporting a caloric deficit during cutting. Milder and slower to act than T3.
Restoring Thyroid Output
Used to replace or top up thyroid output when a diet, a prior aggressive T3 run, or genuine hypothyroidism has left thyroid hormone low.
Smoother Alternative to T3
Because it must convert to T3 and the conversion is self-regulating, T4 gives a more physiological, less spiky stimulus, some prefer it for milder, longer-run use or in a blended T4/T3 protocol.
Benefits
- Raises metabolic rate to support fat loss
- Smoother, more physiological signal than T3 (regulated conversion)
- Very long half-life: stable blood levels, once-daily dosing
- The exact hormone the thyroid normally makes (true replacement)
- Not a controlled substance and not WADA prohibited
- Milder cardiac stimulation than equivalent T3 at sensible doses
Good to Know
T4 is a prohormone - it must convert to T3
T4 is largely inactive on its own; deiodinase enzymes (selenium-dependent) convert it into T3, the hormone that actually drives metabolism. This conversion step is why T4 is slower and milder, and why some people who convert poorly get less from it.
Why lifters usually reach for T3, not T4
For aggressive contest-prep fat loss, T3 is the tool of choice because it is ~4x more potent and acts within hours. T4 is the gentler, more physiological option - slower onset, self-regulating conversion, smoother blood levels - and is often used to maintain thyroid output or in a blended T4/T3 protocol.
It suppresses the thyroid axis, not the testosterone axis
Exogenous T4 lowers TSH and your own thyroid output via feedback, so you must taper and let it recover - but it does not touch the HPTA, so no testosterone PCT is involved.
Not muscle-sparing at high doses
Supraphysiological thyroid levels increase protein turnover and become catabolic, the same as T3 (just milder mcg-for-mcg). Keep protein high and doses sensible.
Not WADA prohibited
Thyroid hormones (T3 and T4) are not on the Prohibited List - a notable contrast with most fat-loss drugs (clen, DNP, tesofensine) that are banned.
Absorption is easily sabotaged
Take it fasted and keep it 3-4 hours away from calcium, iron, magnesium and coffee, all of which markedly reduce absorption. Inconsistent timing makes dosing unpredictable.
Dosage Guidelines
| Experience Level | Dosage Range |
|---|---|
| Beginner | 100 – 150 mcg/day |
| Intermediate | 150 – 200 mcg/day |
| Advanced | 200 – 300 mcg/day |
Full thyroid replacement is ~1.6 mcg/kg/day (roughly 100-150 mcg for most people); clinical dosing for TSH suppression in thyroid-cancer patients generally stays in the same neighborhood. Per the FDA label, doses above 200 mcg/day are "seldom required" and an inadequate response above 300 mcg/day is "rare" (usually signals malabsorption or non-compliance, not a valid reason to push higher), so the advanced figure here should be read as an outer edge, not a target. Physique users run at or somewhat above the replacement range for a metabolic push. Because roughly 3-4 mcg of T4 is needed to match the cellular effect of ~1 mcg of T3, T4 doses look large next to T3 doses. Onset is slow (days to weeks) and effects build to steady-state over about a month (4-6 weeks), so titrate gradually and taper off rather than stopping abruptly. Take fasted; separate from calcium, iron, magnesium and coffee by 3-4 hours, as they blunt absorption. Some run a blended T4/T3 protocol (commonly ~80% T4 / 20% T3) for a middle ground. Monitor with bloodwork (TSH, free T3, free T4). Note: doses beyond the clinical replacement range are physique-community practice extrapolated from T4/T3 potency ratios rather than a bodybuilding-specific clinical trial, treat the intermediate/advanced figures as community-derived, not prescribing guidance.
Roughly 6-7 days in euthyroid individuals: a very long half-life; steady-state takes about 4-6 weeks.
Side Effects
Palpitations / Tachycardia
commonExcess thyroid hormone raises heart rate and can cause palpitations; sustained overdose can provoke atrial fibrillation, especially in older users or with stimulants like clenbuterol/ephedrine. Milder than an equivalent T3 dose but still the main cardiac concern.
Titrate slowly. Monitor resting heart rate. A beta-blocker (e.g. propranolol) can control symptoms. Do not stack with other cardiac stimulants without caution.
Thyroid Axis Suppression
commonExogenous T4 suppresses TSH and endogenous thyroid output via negative feedback. Longer/higher use blunts the body's own production, and abrupt cessation can leave you transiently hypothyroid (sluggish, cold, weight rebound).
Keep runs time-limited, taper down at the end, and allow the thyroid axis to recover. Confirm recovery with TSH/free-T4 bloodwork.
Muscle Catabolism (high doses)
uncommonSupraphysiological thyroid levels increase protein turnover and can eat into muscle, particularly in a deficit. Less pronounced than with high-dose T3 but the same mechanism.
Keep doses sensible, keep protein high (1.6-2.2 g/kg), and avoid extreme deficits. Anabolics offset the catabolic effect.
Bone Density Loss (long-term/high-dose)
rareDecreased bone mineral density is a recognized adverse effect of levothyroxine, driven by chronic TSH suppression from excess thyroid hormone. Risk is low at replacement doses and rises with higher doses, longer duration, and persistently suppressed TSH.
Avoid unnecessarily high doses or indefinite suppression. Ensure adequate calcium/vitamin D intake and weight-bearing exercise. More relevant to prolonged/repeated use than a single short cutting run. Monitor bone density with extended use.
Heat Intolerance / Sweating
commonIncreased thermogenesis causes sweating and difficulty tolerating heat, a sign the metabolic rate is elevated.
Stay hydrated and cool. Reduce dose if excessive.
Anxiety / Tremor / Insomnia
uncommonOver-replacement produces hyperthyroid-type nervousness, fine tremor and disturbed sleep.
Reduce dose. Dose in the morning to limit sleep disruption.
General Mitigation Strategies
T4 is generally milder than T3 because of the buffered conversion step, but the same rules apply: titrate up slowly, do not stop abruptly (taper to let TSH recover), keep protein high to protect muscle, and monitor heart rate. Get periodic bloodwork (TSH, free T3, free T4). Take fasted and away from minerals/coffee for reliable absorption. Be cautious combining with beta-agonists (clen) or other stimulants.
Support Supplements
Ancillary supplements commonly run alongside T4 (Levothyroxine) to manage side effects, support the target tissue, or fill nutrient demands it creates.
Selenium
The deiodinase enzymes that convert T4 into active T3 are selenium-dependent. Adequate selenium supports efficient conversion (relevant because T4 relies entirely on that step to work).
- Dose
- 100-200 mcg/day
- Timing
- Daily with food
- When
- Useful insurance, especially if dietary selenium is low; do not megadose (excess selenium is toxic).
Beta-blocker (e.g. Propranolol)
Controls thyroid-driven tachycardia, palpitations and tremor at higher doses.
- Dose
- As prescribed (low dose)
- Timing
- As needed for symptom control
- When
- Only if heart rate/palpitations become a problem. Note propranolol also modestly inhibits T4-to-T3 conversion, which can blunt the metabolic effect. Prescription item.
Potassium & Taurine
Support electrolyte balance and reduce cramping/palpitation-type symptoms during elevated metabolic states, particularly if stacked with a beta-agonist.
- Dose
- Taurine 3-5 g/day; potassium from diet/electrolytes
- Timing
- Daily; taurine can be split
- When
- Most relevant when combined with clenbuterol-type stimulants that deplete taurine/potassium.
Post Cycle Therapy (PCT)
T4 does not affect the HPTA (testosterone) axis, so no traditional PCT is needed. It DOES suppress the hypothalamic-pituitary-thyroid axis. Do not stop abruptly; taper down and allow the thyroid to recover, verifying with TSH/free-T4 bloodwork.
How It Works
T4 (thyroxine, tetraiodothyronine) is converted by type 1 and type 2 deiodinases (5'-deiodinase, a selenium-dependent enzyme) into T3 in the liver, kidney, muscle and other tissues. T3 is the metabolically active hormone that enters the nucleus, binds thyroid hormone receptors on DNA and up-regulates transcription, increasing basal metabolic rate, cellular oxidation, thermogenesis and catecholamine sensitivity. Because the conversion step is regulated (the body can shunt T4 toward inactive reverse-T3), circulating T4 gives a smoother, more buffered metabolic signal than dosing T3 directly.
Fundamentals
Reference on the practices relevant to T4 (Levothyroxine): how they are done and where they go wrong. Not a recommendation to use it.
Common Stacks
- T4 + Testosterone (or another anabolic) - Metabolic push while protecting muscle
- Blended T4/T3 (~80/20) - Smoother thyroid protocol vs. T3 alone
- T4 + Clenbuterol - Cutting stack (monitor heart rate; supplement electrolytes)
Legal Status
FDA-approved prescription medication for hypothyroidism (USA). Not a controlled substance and not WADA prohibited.
Legal status varies by country and changes over time. This is a general summary, not legal advice.
WADA Status
T4 (levothyroxine/thyroxine) is not on the WADA Prohibited List. Thyroid hormones are permitted in and out of competition. (WADA has periodically discussed monitoring thyroid hormones but they remain non-prohibited.)