L-Thyroxine

L-thyroxine (levothyroxine) is a medication for treating an underactive thyroid (hypothyroidism). It contains synthetic thyroxine (T4), which corresponds to the thyroid hormone the body produces itself.

In hypothyroidism, the thyroid gland produces too little hormone. L-thyroxine replaces the missing hormone and normalizes the metabolism. It is usually taken for life.

L-thyroxine is available under numerous brand names, including L-Thyroxin Henning®, L-Thyrox® HEXAL®, Euthyrox®, Eferox® and Levothyroxin Abdi. The products differ in their inactive ingredients and cannot be switched arbitrarily.

Levothyroxine is the synthetically produced form of thyroxine (T4) — the hormone that the thyroid gland mainly produces. The body cannot distinguish it from its own hormone.

T4 is a storage form. The body converts it into active T3 as needed, right where it is needed. This results in a steady hormone level that hardly fluctuates over the course of the day.

Thyroid hormones control the metabolism: heat production, heart rate, digestion, concentration, mood. With an underactive thyroid, these processes slow down — fatigue, feeling cold, weight gain and lack of drive are typical. Treatment replaces the missing hormone.

The active ingredient has a narrow therapeutic range: even small differences in the amount absorbed can shift blood test results and how you feel. That is why the exact dose matters, and why switching products is not merely an administrative matter.

L-thyroxine is suitable for:

• Hypothyroidism (underactive thyroid) of any cause
• Hashimoto's thyroiditis
• After thyroid surgery (partial or complete)
• After radioactive iodine therapy
• Goiter prophylaxis (prevention of goiter)
• Suppression therapy in thyroid cancer
• Congenital hypothyroidism

The most common cause in Germany: Hashimoto's thyroiditis – an autoimmune disease in which the immune system attacks the thyroid gland.

Available strengths:

• 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 mcg

Dosage:

• Highly individual – based on TSH level, age, weight and coexisting conditions
• Starting dose in adults: usually 25-50 mcg/day
• Increase by 25 mcg every 4-6 weeks
• Usual maintenance dose: 75-150 mcg/day
• In older patients/heart disease: start more cautiously (12.5-25 mcg)

TSH target: Usually 0.4-2.5 mU/l (to be set individually).

• Once daily in the morning, on an empty stomach, at least half an hour before breakfast, with a glass of water.
• The time gap before the meal is crucial. Coffee, milk, soy products and high-fiber foods significantly reduce absorption.
• The dose is set by the doctor and usually increased gradually.
• Do not change or stop it on your own. The effect builds up over weeks.

Intervals from other medicines

• At least two hours from products containing calcium, magnesium or iron, as well as from heartburn remedies.
• At least four to five hours from colestyramine and similar agents.

Missed dose: If you notice it the same morning, you can still take the tablet. Otherwise, continue as normal the next day — do not take a double dose.

After switching products, blood levels should be checked, usually after six to eight weeks. This also applies when switching between products with the same amount of active ingredient.

Do not use in cases of:

• Hypersensitivity to levothyroxine or any of the other ingredients.
• Untreated overactive thyroid.
• Untreated adrenocortical insufficiency.
• Untreated dysfunction of the pituitary gland with adrenal involvement.
• Recent heart attack, acute inflammation of the heart muscle, acute inflammation of all layers of the heart wall.

To be clarified with a doctor before use: Coronary heart disease, heart failure, heart rhythm disorders, high blood pressure, advanced age, and a long-standing underactive thyroid. In these cases, treatment is started at a low dose and increased slowly.

During pregnancy, treatment is continued; the requirement often increases. Blood levels should be monitored more closely. Do not use together with medicines for an overactive thyroid.

Not for weight loss. In people with normal thyroid function, higher doses can cause serious, even life-threatening side effects.

When the dose is correctly adjusted, side effects are rare. If they occur, they are usually signs of a dose that is too high:

• Palpitations, fast or irregular pulse
• Inner restlessness, nervousness, tremor
• Sleep disturbances
• Increased sweating, heat sensitivity
• Weight loss despite unchanged eating habits
• Diarrhea
• Headache, muscle weakness, muscle cramps
• In women: menstrual cycle disorders

Such signs should be evaluated by a doctor; usually, adjusting the dose is sufficient.

Rare: Hypersensitivity reactions of the skin, such as rash or hives. With prolonged use of higher doses, bone density may decrease.

Have it checked by a doctor immediately in case of: persistent racing heart, heart rhythm disorders, chest pain or shortness of breath.

Absorption is reduced by:

• Products containing calcium, magnesium or iron; heartburn remedies
• Acid blockers (proton pump inhibitors such as pantoprazole or omeprazole) — they change the pH in the stomach; when starting or stopping such treatment, thyroid levels should be checked
• Colestyramine, colestipol, sevelamer, lanthanum carbonate
• Sucralfate, orlistat
• Soy-containing foods and dietary supplements
• Coffee, if it is drunk too close to taking the medication

Absorption is increased by:

• Semaglutide: Using both at the same time can affect the amount absorbed. After a single dose of semaglutide taken by mouth, it increased by about one third. Anyone receiving both needs closer monitoring of thyroid levels and, if necessary, a dose adjustment.

Breakdown is accelerated by: Carbamazepine, phenytoin, phenobarbital, rifampicin.

The effect of other medicines may change:

• Coumarin-type blood thinners: The tendency to bleed may increase; clotting values should be checked.
• Blood sugar-lowering medicines and insulin: The requirement may change; blood sugar should be monitored more closely, especially at the start of treatment.

Other: Amiodarone, iodine-containing contrast agents and lithium affect thyroid function. Estrogen-containing products can increase the requirement. Salicylates and high doses of furosemide can shift hormone levels in the blood. Certain cancer medicines (tyrosine kinase inhibitors) and HIV medicines (protease inhibitors) can weaken the effect.

Before a blood test: Biotin — contained in many dietary supplements for skin and hair — can distort the measurement of thyroid levels. Please mention this beforehand if you take biotin.

Please list all medicines you take, including non-prescription ones.