Levothyroxine vs Synthroid: Benefits, Side Effects, Dosage, and Interactions

In this article, we will take a close look at Levothyroxine and Synthroid, two medications used for thyroid hormone replacement. We will discuss their uses, benefits, potential side effects, dosage, and interactions. We will also compare the differences between these two drugs and their role in treating conditions like hypothyroidism and certain types of thyroid cancer.
Nithishwer Mouroug Anand

Nithishwer Mouroug Anand

Nithish is a computational biochemist at the University of Oxford working on alchemical methods for protein-drug interactions.

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Levothyroxine vs Synthroid

Levothyroxine is a generic drug, while Synthroid is a brand-name drug. Both contain the same active ingredient, levothyroxine, which works by replacing the thyroid hormone that your body would typically produce [1, 2, 3].

Even though they have the same active ingredient, there can be differences in their bioavailability. One study found that Synthroid treatment leads to a significantly higher free thyroxine level than those treated with Levothroid, a brand of levothyroxine [4]. However, other studies have shown no significant differences in clinical scores or thyroid hormone measurements between patients treated with Synthroid or other brands of levothyroxine [5].

Cost-wise, generic drugs like levothyroxine cost less than brand-name drugs like Synthroid [3]. But, switching to a cheaper alternative without consulting your physician should be avoided as it can have potential side effects [3].

Uses of Levothyroxine and Synthroid

Treating Hypothyroidism and Cancer: Levothyroxine and Synthroid are primarily used to treat hypothyroidism. The individuals with this condition have low thyroid hormone levels due to underproduction by the thyroid gland. These medications act as a replacement for your body's thyroid hormone, helping restore normal physiological conditions [1, 6].

Moreover, these drugs are also used to treat certain types of thyroid cancer. They are usually prescribed as part of a treatment plan, including radioactive iodine therapy and surgery [1, 6].

Synthroid is used for thyroid-stimulating hormone (TSH) suppression in adults and some children [3].

Treating goitre and NAFLD: Levothyroxine is also used to treat goitre, an enlarged thyroid gland, and it has been found to have a crucial role in reducing the effects of Non-Alcoholic Fatty Liver Disease (NAFLD). This condition can occur due to an underactive thyroid gland[2, 7].

Used in Combination therapy: In some cases, levothyroxine is combined with other medications as a combination therapy [2]. For instance, some patients who do not fully benefit from levothyroxine alone have greatly benefitted by switching to a combination of levothyroxine and liothyronine (LT4+LT3) [8].

Levothyroxine and Synthroid Side Effects

Levothyroxine and Synthroid, both thyroid hormone replacement medications, can cause a range of side effects.

Common side effects: These include weight changes, headaches, vomiting, diarrhoea, changes in appetite, fever, menstrual cycle changes, sensitivity to heat, hair loss, joint pain, and leg cramps [6].

Serious side effects: These can include shortness of breath, wheezing, hives, itching, rash, flushing, stomach pain, nausea, or swelling of hands, feet, ankles, or lower legs [6]. In a few cases, levothyroxine can also cause increased blood sugar levels, decreased bone mineral density, heart-related problems such as heart palpitations, and severe allergic reactions [9].

Synthroid can cause similar side effects. These can include headaches, irregular heartbeat, muscle weakness or spasms, nervousness or anxiety, and irregular periods [10].

A few long-term side effects that these medications can cause include weakened bones, heart damage, and, in some cases, shortened height as an adult [11].

Effects in children: Especially Synthroid can cause various effects in children like increased pressure inside the skull, hip dislocation, early closure of fontanelles affecting head shape and brain growth, and early closure of growth plates affecting height as an adult [10].

Levothyroxine vs Synthroid: Interactions

Levothyroxine can interact with certain medications, leading to changes in the body's response. For instance, iron, calcium, proton pump inhibitors, and oestrogen can increase serum TSH concentration, potentially reducing the effectiveness of levothyroxine [12]. Other medications like antiseizure drugs, blood thinners, birth control pills, cholesterol-lowering medicines, oestrogen derivatives, and certain antibiotics can also potentially interact with levothyroxine.

Moreover, antidepressants such as amitriptyline and maprotiline can increase the side effects of levothyroxine, potentially leading to irregular heart rhythms. Similarly, sympathomimetic drugs such as pseudoephedrine and albuterol can amplify the effects of both drugs, which may lead to serious heart problems [2].

Levothyroxine vs Synthroid: Dosage

The dosage of levothyroxine varies depending on the individual's age, weight, and medical condition. It's crucial to take levothyroxine as directed by your healthcare provider. Some medications may require you to adjust your levothyroxine dosage. For instance, if you're taking the antidepressant sertraline, with levothyroxine your doctor might need to increase your levothyroxine dosage [2].

It's important to note that certain substances can make levothyroxine less effective by decreasing its amount in the body. These include calcium carbonate, ferrous sulphate, and certain cholesterol-lowering medications. To guarantee proper working of levothyroxine, it's best to take it 4 hours before taking any other medications.[2].

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Nithishwer Mouroug Anand

Nithishwer Mouroug Anand

Nithish is a Doctoral Researcher in Computational Biochemistry at the University of Oxford. A physicist by training, he applies principles of thermodynamics and computational methods to investigate the interactions between drugs, proteins, and cell membranes. His expertise ranges from single-cell RNA sequencing and cancer genomics to utilizing free energy methods to understand protein biophysics.