Levothyroxine (L-thyroxine, T4)

What levothyroxine is

Levothyroxine, L-thyroxine for short, is the synthetic form of the thyroid hormone thyroxine (T4). It is prescription-only and, measured by prescribed daily doses, one of the ten most prescribed medicines in Germany, alongside drugs for high blood pressure, stomach acid and high cholesterol (Schröder 2025). It is used above all when the thyroid produces too little hormone, whatever the cause.

Levothyroxine also has a second reputation: as a remedy for tiredness with borderline lab values, for weight loss and in bodybuilding, to raise the basal metabolic rate during the diet phase. The more potent active hormone liothyronine (T3) has its own page, as does iodine, the building block of thyroid hormones.

How it works

Levothyroxine is chemically identical to the T4 of the thyroid. T4 is mainly a precursor: about 80 percent of the active hormone T3 in the blood is formed outside the thyroid, mainly in the liver, when T4 gives up an iodine atom. T3 binds to receptors in the cell nucleus and controls the reading of genes; this raises the basal metabolic rate as well as sugar, fat and protein metabolism, and affects the heart, bones and brain (US prescribing information).

T4 has a half-life of 6 to 7 days and therefore acts evenly. Taken from outside, it lowers the TSH of the pituitary gland and with it the body's own hormone production; the dose can be set by a physician via TSH. The idea behind the misuse is the same metabolic boost: higher basal metabolic rate, more fat loss. But it also hits the heart and bones.

What the studies show

TRUST 2017: mildly raised TSH in old age

737 people aged 65 and over, mean age 74.4 years, with persistently mildly raised TSH (4.60 to 19.99 mIU/l) and normal free T4 received, double-blind, levothyroxine or placebo; the dose was adjusted according to TSH, with mock adjustment in the placebo arm. After one year, TSH fell to 3.63 versus 5.48 mIU/l with levothyroxine. The hypothyroid symptom score changed by 0.2 points in both groups, the tiredness score by 3.8 versus 3.2 points. Secondary outcomes showed no benefit either; serious adverse events were not more frequent.

Meta-analysis 2018 and BMJ guideline 2019

Feller and colleagues analyzed 21 randomized trials with 2,192 non-pregnant adults with subclinical hypothyroidism. TSH fell into the normal range, but quality of life and thyroid-related symptoms did not improve. On this basis, a guideline panel in the BMJ in 2019 issued a strong recommendation against thyroid hormones in subclinical hypothyroidism, finding no relevant benefit for tiredness, depressive symptoms or body weight either. Exceptions are women trying to become pregnant and people with TSH above 20 mIU/l; it may not apply to young adults and severe symptoms.

Weight loss: what the studies say about weight

A systematic review examined T3 and T4 in people with obesity during calorie restriction. A consistent effect on weight loss, protein breakdown or basal metabolic rate could not be shown, but an artificial overactive thyroid in lab values could (Kaptein 2009). The German product information states explicitly that normal doses do not reduce weight when the thyroid is healthy.

What users report: misuse in sport and dieting

A systematic review of poisoning cases over the last 30 years found that thyroid hormones are misused mainly by athletes, by people trying to lose weight and in suicide attempts. Doping was the most common cause, often together with anabolic steroids, and cases have been rising since 2015 (Skrzypiec-Spring 2025). Among Australian elite athletes, by contrast, use was rare: in doping samples and control forms it was about 4 per 1,000 athletes (Handelsman 2023).

Where the data stop

• A benefit in mildly raised TSH. In subclinical hypothyroidism, the large trials show no benefit for symptoms and quality of life. Whether young adults or people with severe symptoms benefit is left open by the BMJ guideline.
• Hard endpoints in subclinical hypothyroidism. According to the BMJ guideline, the data on cardiovascular events and mortality are weak, and harms were recorded in only one trial with few events.
• Weight loss. A weight effect with a healthy thyroid has not been shown. The existing studies are small and of limited quality.
• The consequences of misuse. How often levothyroxine is used in recreational and strength sports and how often it causes harm there has hardly been studied. There are case reports, but no systematic data on users outside elite-sport testing.

Status, approval and legal

In Germany, levothyroxine is an approved, prescription-only medicine; there are numerous products. It is approved, among other uses, for an underactive thyroid of any cause, for preventing recurrence after goiter surgery, for benign goiter with normal function and for thyroid cancer. The product information states: thyroid hormones must not be given for weight reduction.

In the USA, the prescribing information (for example Synthroid) carries a boxed warning: not for the treatment of obesity or for weight loss; doses above the daily requirement can cause serious or life-threatening toxicity. In sport, thyroid hormones are not prohibited under the WADA Code (Handelsman 2023). Whether and at what dose to treat is decided by the physician based on TSH and symptoms.

Safety

At the right dose, levothyroxine is well tolerated because it replaces a hormone the body makes itself; according to the US prescribing information, allergy to the active substance itself is not known, though allergy to excipients is. Side effects arise mainly from too much hormone: palpitations, rapid heartbeat, rhythm disturbances, insomnia, nervousness and headache. Too high a starting dose can trigger atrial fibrillation in sensitive people; in older people and those with heart disease, treatment is therefore started lower (product information).

Over-replacement over a long time increases bone breakdown and lowers bone density. In people over 70, current use was associated with a higher fracture risk in a study of 213,511 users, with a clear dose-response: odds ratio 3.45 for high versus low dose (Turner 2011). For misuse at higher amounts, especially with appetite suppressants, the product information warns of life-threatening consequences; deaths have been described (Skrzypiec-Spring 2025).

Sources

• Stott DJ et al., N Engl J Med 2017 – TRUST, levothyroxine in subclinical hypothyroidism, 737 people aged 65 and over
• Feller M et al., JAMA 2018 – meta-analysis, 21 randomized trials with 2,192 adults with subclinical hypothyroidism
• Bekkering GE et al., BMJ 2019 – guideline: thyroid hormones in subclinical hypothyroidism
• Kaptein EM et al., J Clin Endocrinol Metab 2009 – thyroid hormone in obesity and other illnesses, systematic review
• Turner MR et al., BMJ 2011 – levothyroxine dose and fracture risk in people over 70, 213,511 users
• Schröder L, Seifert R, Naunyn Schmiedebergs Arch Pharmacol 2025 – the ten most prescribed medicines in Germany 1985 to 2022
• Skrzypiec-Spring M et al., Pharmaceuticals 2025 – poisoning from illegally used thyroid hormones, systematic review
• Handelsman DJ et al., J Endocr Soc 2023 – thyroid hormone abuse among elite athletes, not prohibited by WADA
• German product information (Fachinformation) L-Thyroxin Winthrop (Sanofi, as of February 2023)
• US prescribing information Synthroid (DailyMed) – boxed warning: not for weight loss