Levothryoxine (also marketed as as Synthroid, Levoxyl, Levothroid, Unithroid) is prescribed for hypothyroid conditions and to suppress thyroid hormone release in the management of cancerous thyroid nodules and growth of goiters.
The tablets themselves contain T4, an inactive form of hormone which is normally produced by the thyroid. This is converted by other tissues within the body into a hormone known as T3, which is the more active component, and regulates cellular metabolic activities - in other words, the energy and heat levels of the cells and body.
How Does Levothyroxine (Synthroid) Work?
When the thyroid is functioning correctly, the hypothalamus (in the brain) regulates how much T3 is circulating in the body. If it senses that the levels are low, it releases TRH (thyrotropin-releasing hormone) to tell the pituitary gland (also in the brain) to release TSH (thyroid-stimulating hormone). This stimulates the thyroid gland to produce T4, by combining iodine and tyrosine (an amino acid found in the body).
Once T4 is in the blood, it is converted into T3 within the peripheral tissues - mainly the liver and kidneys - using cortisol (a hormone secreted by the adrenal glands). The T3 enters the body's cells, connects to the right receptors, and the energy is released.
What Can Stop the Conversion of T4 to T3?
The body can only convert to T3 if:
- the liver and kidneys have enough cortisol to cause the conversion. If the adrenal glands' store of cortisone is depleted, conversion cannot occur. This means that anyone prescribed Thyroxine should ideally have their adrenal levels tested before taking the drug. See: Hypothyrodism and Adrenal Insufficiency.
- the T4 in the blood avoids cytokines - messenger cells that direct immune function and help cells communicate, some of which can block the conversion of T4 to T3.
Can Anything Else Stop T3 From Working?
Even if the T4 is converted to T3, it must still enter the body's cells and connect to the right receptors to affect their metabolic function. If the cellular membranes or receptor sites are not functioning correctly, the T3 cannot perform.
Can any Foods Stop Levothyroxine (Synthroid) From Working?
Levothyroxine (Synthroid) is absorbed from the gut, and its absorption is increased during the "fasting state" - i.e. at least one hour before food, or three hours after eating. For this reason, many people prefer to take their tablets before breakfast, or last thing at night. If the tablets are taken with food, a higher dosage of Thyroxine has to be taken to maintain the same level of drug availability for the body.
Some substances bind with the Levothyroxine (Synthroid), so that it cannot cross the gut wall to be absorbed. These substances should never, therefore, be taken at the same time, or within an hour of taking Thyroxine tablets: iron salts, antacids, calcium carbonate (including milk), sucralfate, cholestyramine, and soy-based formulas. Leave at least four hours between taking calcium or iron supplements, or soy products, and a dose of Thyroxine.
Recent research suggests that caffeine may also prevent full absorption of Levothyroxine (Synthroid).
How Should I Take my Synthroid?
There are 3 really important things to remember when taking Levothyroxine:
- You must not have eaten any food for at least two hours, to gain the full effect. For this reason most people take their tablets before breakfast or last thing at night.
- The majority of the levothyroxine dose is absorbed from the small intestine (the jejunum and upper ileum).1 To "propel" the pill down to the jejunum you need to drink approximately 300ml (around 1/2 pint) of water. This is essential - the drug will not be adequately absorbed from other sites in the body.
- You must not take calcium or iron supplements within four hours of taking Synthroid or any form of Levothyroxine. The thyroxine binds to the iron or calcium and leaves the body without having had any effect.
Is Selenium Important?
Selenium is a component that helps convert T4 to T3, so deficiencies of selenium can impair thyroid function and worsen hypothyroidism. However, selenium can be toxic in large quantities, and some studies have shown that an excess intake of selenium can actually depress T3 levels. It can also lead to higher cholesterol levels and even coronary heart conditions (read more here).
Research suggests that 200mcg per day is the ideal level at which to supplement selenium in hypothyroid and Hashimoto patients.
See also: Vitamins and supplements for Hypothyroidism
What About Iodine?
Thyroid expert Mary Shomon points out that many forms of thyroid disease are nothing to do with iodine, and iodine deficiency is not as common as it is sometimes said to be. Thyroids are extremely sensitive to iodine, and you need to be careful about adding too much iodine to the diet as it can irritate or aggravate the thyroid.
Iodine or kelp supplements are particularly dangerous for people with auto-immune (Hashimoto's) thyroid disease. See Iodine and Hashimoto's Thyroiditis.
What Is the Best Way to Take Synthroid (Levothyroxine)?
The key thing is consistency: take the same dosage at the same time, with the same interval from foods, each day. This ensures that your blood tests reflect the true situation within your body, and your doctor is then in a better position to prescribe the correct drugs and dosages.
Take the following medicines and supplements at least 4 hours before or 4 hours after you take levothyroxine: antacids, calcium (as milk, supplements, or antacids), cholestyramine, colestipol, iron, simethicone, sodium polystrene sulfonate, or sucralfate,
Drug Interractions and Synthroid
Levothyroxine (Synthroid) can affect, or be affected by insulin (used by diabetics) and warfarin (a blood-thinning drug). The fact that it raises the body from a hypothyroid state to a euthroid (normal) one, means that it affects other systems within the body, so the dosage of heart medicines such as beta-blockers may need to be adjusted can decrease the actions of some beta-blockers.
Cholesterol-lowering drugs can decrease the effect of Synthroid (the Levothyroxine binds to the cholesterol drug and so is not absorbed by the body).
References
Sarah Tomley - Sarah is an editor and writer who has been working in the UK publishing industry for over 18 years. She has an honours degree in ...
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