Levothyroxine (Synthroid)

Hello friend!

I hope your year has started well. Time to look at another drug. Levothyroxine, marketed under brand names Synthroid and Levoxyl, is synthetically manufactured thyroxine (also called T4). T4 is the major hormone secreted by the thyroid gland.

The thyroid hormone is made in the body by the thyroid gland, which is located in the neck. What does the hormone do? What is its role in the body? It plays an important role in metabolism. It is a catalyst in oxidative reactions. The hormone increases oxygen and energy consumption of the body – it increases the basal metabolic rate (BMR)  – BMR is the minimal calorie requirement of the body in resting state. The higher the BMR, the higher the number of calories burnt. This has resulted in a misuse of Levothyroxine for weight loss. The thyroid hormone affects body growth, temperature, heart rate and the force of contraction of the heart.

To be more accurate, thyroid hormones (plural) are secreted by the thyroid gland. The significant thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Iodine is an essential part of the synthesis of the thyroid hormones. Another hormone called thyroid stimulating hormone (TSH) is synthesized and secreted by the pituitary gland in the brain. TSH stimulates the thyroid gland to release T3 and T4. The release of TSH is controlled by negative feedback – the higher the levels of T3 and T4 in the blood, the lower the TSH levels. And, the lower the T3 and T4 levels, the higher the TSH levels. TSH levels can be easily measured in the laboratory, and because of the easy correlation with T3 and T4, TSH is important to diagnose hyperthyroidism or hypothyroidism.

Hypothyroidism is a result of insufficient production of thyroid hormone by the thyroid gland. About three to five per cent of the U.S. population suffer from this condition. There have been reports that as much as ten per cent of the population may be affected if the mildest forms of subclinical hypothyroidism is included. This has a larger implication, because hypothyroidism can result in higher levels of cholesterol in the body, that could result in cardiovascular problems.

Iodine deficiency in diet can cause hypothyroidism – this can be a problem worldwide, however it is not significant in the U.S. Hypothyroidism can also be caused by thyroiditis (inflammation of the thyroid gland). How does the thyroid gland get inflamed? There are many possible reasons  ….. but it typically comes down to this: the body’s immune system …… well, it goes a little crazy. The normal role of the immune system is to develop antibodies against and attack foreign cells like bacteria and viruses. In thyroiditis, the immune system develops antibodies against the cells of the thyroid gland and attacks the thyroid gland cells in a self-destructive, autoimmune type activity.

Hashimoto’s thyroiditis or chronic lymphocytic thyroiditis results from the T-cells of the immune system attacking the thyroid. Post-partum thyroiditis affects about 5% women within one year of giving birth – the symptoms presented are first hyperthyroidism, then it gets back to normal or in about 20 per cent of those affected, it moves further into life-long hypothyroidism. Some kinds of thyroiditis can be genetically acquired. In some individuals, drugs like amiodarone (for the heart) and the anti-viral interferon can attack the thyroid cells and cause thyroiditis.

Common symptoms of hypothyroidism include fatigue, drowsiness, weight gain, increased sensitivity to cold, muscle cramps, low pulse rate, depression and constipation.

Typically the first laboratory test to evaluate hypothyroidism is measuring TSH. The higher the TSH, the greater the hypothyroidism (i.e., the lower the levels of thyroid hormone). The normal range for adults is usually in the range of 0.3 to 5 mIU/L – there are some variations between laboratories where the test is done.

The medication usually prescribed for hypothyroidism is Levothyroxine. It is available in various strengths 25 micrograms to 200 micrograms. It is usually taken on an empty stomach a half hour to one hour before breakfast, with a full glass of water. Antacids that contain calcium, aluminum or magnesium, simethicone, calcium supplements, iron, cholestyramine, colestipol, sucralfate and sodium polystyrene sulfate bind to levothyroxine and reduce the absorption from the stomach into the bloodstream. Therefore none of these listed medications should be taken less than 4 hours from levothyroxine ingestion.

Rarely, there could be temporary hair loss in the first few months of starting levothyroxine. Otherwise, it is a well-tolerated drug. Side-effects often occur if the dosage taken is too high – in this case, symptoms of hyperthyroidism are seen. These include increased sweating, increased appetite and weight loss, fever, increased pulse rate – in extreme causes even leading to a heart attack, high blood pressure and seizures. Side-effects of overdosing with levothyroxine occur 6 hours to 11 days after ingestion.

Every few months, TSH levels are tested to make sure that the dosage of medication is correctly titrated.

With that ……. I hope you have a wonderful week; wish you good health!

Dr. Ajit Damodaran


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