Levothyroxine
Levothyroxine (Synthroid) replaces the thyroid hormone T4 in people whose gland underproduces it. The product comes in seven strengths, from 25 mcg to 200 mcg, for Hypothyroidism and Thyroid-stimulating Hormone Suppression.
Key points
- Levothyroxine must not be used for weight loss; in people with normal thyroid function, doses within daily requirements are ineffective, and larger doses can cause serious or life-threatening toxicity.
- Levothyroxine sodium has a narrow therapeutic index, so both under- and over-treatment carry risks to heart, bone and metabolic function.
- For secondary or tertiary hypothyroidism, a TSH blood test is not a reliable way to judge whether the dose is right.
How does Levothyroxine work?
Levothyroxine is synthetic T4, the hormone your thyroid gland normally makes. Once absorbed, about 80% of it converts in your tissues into T3, the form that drives cellular activity. T3 enters the nucleus of your cells and binds to receptor proteins attached to DNA, switching on the genes that control how fast your body burns energy and builds proteins. An underactive thyroid slows the body down; replacement restores normal metabolic rate. Getting the dose right is central to Thyroid Health.
Further reading
What is levothyroxine used for?
Levothyroxine replaces the thyroid hormone T4 in people whose thyroid gland underproduces it. It is used for hypothyroidism and for thyroid-stimulating hormone suppression.
How does levothyroxine work in the body?
Levothyroxine is synthetic T4, the hormone your thyroid gland normally makes. Once absorbed, about 80% of it converts in your tissues into T3, the form that drives cellular activity. T3 enters the nucleus of your cells and binds to receptor proteins attached to DNA, switching on the genes that control how fast your body burns energy and builds proteins.
How long does levothyroxine take to work?
Levothyroxine usually takes 1 to 2 weeks to show its full effect.
What are the side effects of levothyroxine?
Common side effects include headache, sweating and heat intolerance, while moderate side effects can include tachycardia, weight loss and nervousness. Severe cardiac arrhythmia is a serious side effect that requires immediate medical attention. Speak to your doctor or pharmacist if you are concerned.
Can levothyroxine be taken with other medicines?
Levothyroxine can interact with calcium carbonate, iron supplements and warfarin. If you take any of these, ask your doctor or pharmacist for advice.
What levothyroxine products are available?
On Direct Meds, levothyroxine comes as Synthroid. Compare their forms, strengths and packs.
What brand names is levothyroxine sold under?
Levothyroxine is also sold under brand names such as Eltroxin. Names differ from country to country, so the active ingredient is the surest way to compare products.
How much does levothyroxine cost in South Africa?
Compare the rand prices for each product, strength and pack above. Prices are converted from US dollars. Larger packs usually cost less per tablet.
Payment and prices →Can I buy levothyroxine over the counter in South Africa?
Yes, online. Direct Meds does not ask for a prescription to order levothyroxine. We are a broker, not a dispensing pharmacy. We arrange orders and delivery with our affiliated supplier on your behalf.
Ordering without a prescription →Can I buy levothyroxine online in South Africa?
Yes. Our affiliated supplier delivers across South Africa. Estimated delivery is 7 days with Express Delivery, which includes online tracking, or up to 3 weeks with Regular Delivery.
How ordering works →Related articles
22 Sept 2026 ⋅ 5 min read
Still tired on Eltroxin? Where T3 fits in thyroid treatment
Compare levothyroxine (T4) with liothyronine (T3), why symptoms may persist on treatment, and what the evidence says about a supervised combination trial.
Read guide