Thyroid for Dummies (22 page)

ߜ If you have mild subacute thyroiditis (head to Chapter 11), some drugs will make it worse to the point that you experience symptoms.

Identifying the Effects

of Specific Substances

In this section, you encounter particular drugs that affect the thyroid, grouped together according to their potential impact on your thyroid.

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Initiating or aggravating hypothyroidism

Many drugs have the potential to cause or intensify hypothyroidism. The parts of this section look at the most commonly prescribed medications to have this potential side effect.

Drugs that compete with iodine

If iodine can’t enter your thyroid, you can’t make thyroid hormone, and you experience hypothyroidism. Some drugs compete with iodine for entry into the thyroid. Usually the effect is mild, and hypothyroidism doesn’t occur. But if your diet is limited in iodine, these drugs can cause low thyroid function.

The most important drug in this category is lithium, which is used to treat manic-depression. In one study, as many as 10 per cent of people given lithium became hypothyroid. This effect is much more common in women than in men and occurs within the first two years of treatment. Most likely, the large population of women with autoimmune thyroid disease (refer to Chapter 5) is most susceptible to the antithyroid effect of lithium. Not only does lithium block the uptake of iodine, but it also inhibits the production and release of thyroid hormone. Some people taking lithium develop a goitre. Curiously (and rarely), lithium can cause hyperthyroidism rather than hypothyroidism.

The mineral fluorine, which is found mainly in fluoridised water and dental products, has a similar effect on the thyroid. If you consume substantial amounts of fluorine, your thyroid will decrease its production of T4. Your pituitary gland then makes more TSH to stimulate the thyroid, and you can end up with a goitre.

Drugs that prevent the addition of iodine to form thyroid hormones Another group of medications blocks the production of thyroid hormones in a slightly different way. These medications keep iodine from combining with thyronine to form either T4 or T3, the two thyroid hormones. This group of medications includes:

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Aminoglutethimide:
Used in the treatment of breast and prostate cancer, although it’s now largely replaced with newer, better tolerated drugs. As many as one-third of patients treated with this medication develop hypothyroidism.

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Ketoconazole:
Used as an antifungal drug. Hypothyroidism is a rare side effect of this agent.

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Sulfonamide drugs:
Used to eliminate excess water from the body and act as antibiotics, such as sulfadiazine and acetazolamide. If you use 16_031727 ch10.qxp 9/6/06 10:46 PM Page 114

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these diuretics and antibiotics for prolonged periods of time, they can cause hypothyroidism.

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Sulphonylureas:
Used in the treatment of diabetes, such as tolbutamide and chlorpropamide.

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Antithyroid drugs:
Used to treat hypothyroidism, such as propylthiouracil and carbimazole

Two chemicals found in food have an impact similar to the medications in the preceding list to produce hypothyroidism. They are

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Isoflavones:
Found in soybeans. Children fed on large amounts of soy products may develop a goitre.

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Thiocyanate:
Contained in many vegetables, such as:

• Broccoli

• Brussels sprouts

• Cauliflower

• Cabbage

• Horseradish

• Kale

• Kohlrabi

• Mustard

• Swede

• Turnips

If cattle consume foods like these and you drink milk from those cattle, your thyroid is sometimes affected as well.

Drugs that affect the transport of thyroid hormone

Many drugs affect how thyroid hormones are transported in your bloodstream. Chances are you’ll take one of these drugs at some point in your life.

If you have normal thyroid function, your thyroid simply makes more or less thyroid hormone to compensate for the effects of these drugs. However, if you’re hypothyroid and taking a thyroid hormone replacement, you may need to adjust your dose when taking one of these drugs.

The following drugs increase thyroid-binding protein, resulting in an increase in total (but not free) thyroxine unless you get your thyroid hormone as a medication:

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Oestrogens:
The most commonly used drugs in this category. For years, oestrogens caused confusion with respect to thyroid function because doctors used to measure how much total thyroxine was in your system, not just how much free thyroxine is there (refer to Chapter 4). Oestrogens are found in the combined oral contraceptive pill and hormone replacement therapy. Some animals are fed oestrogens to fatten them up, so as you consume those animals, you get oestrogen that way as well. The list of medications that contain oestrogens is huge.

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Clofibrate:
A drug used for lowering blood fats.

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Perphenazine:
A treatment for psychotic disorders in the group of drugs called phenothiazines. This group includes a number of well-known medications, such as prochlorperazine, trifluoperazine, and chlorpromazine.

Perphenazine is the main ingredient in a number of different preparations and is also used for treating nausea and vomiting.

The following drugs decrease thyroid-binding protein resulting in a decrease in total (but not free) thyroxine unless you get your thyroid hormone as a medication:

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Androgens:
A substitute for the male hormone, testosterone, when a man cannot make his own. They permit muscle growth and normal sexual function.

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Corticosteroids:
Used extensively to treat inflammation and to reduce immunity when inflammation and autoimmunity are damaging the body, such as in rheumatoid arthritis and many other illnesses. The list of corticosteroids is a long one.

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Nicotinic acid:
A vitamin used for the treatment of elevated fats in the blood.

If you are hypothyroid and are put on one of these agents for a long time, ensure that your doctor remembers to check your thyroid function periodically.

The opiates heroin and methadone also affect the movement of thyroid hormones in the blood stream.

Amiodarone

Amiodarone is used for disturbances of the heart rhythm. This drug may cause hypothyroidism in up to 10 per cent of the people who take it. The drug is also associated with a number of other side effects, including skin and corneal discolouration and fibrosis of the lungs. It may cause hepatitis and bone marrow suppression. Despite all these negatives, the drug is very useful in treating heart rhythm disturbances.

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Drugs used to treat severe hyperthyroidism

The drugs in this group are useful when an individual has severe hyperthyroidism and needs to reduce the T3 hormone level as soon as possible.

However, these drugs have other primary purposes, and when they are prescribed for those other purposes (to patients who aren’t hyperthyroid), they can create hypothyroidism.

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Corticosteroids:
These drugs are discussed in the previous section.

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Iodinated contrast agents:
These drugs are used for achieving better X-ray studies. A single dose can last for 10 days.

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Propranolol:
Used to slow a rapid heartbeat, but it’s also used to treat hyperthyroidism because it controls many of the symptoms, such as palpitations, shakiness, and nervousness.

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Propylthiouracil:
One of the standard drugs for the treatment of hyperthyroidism in the United Kingdom.

Growth hormone

When the body does not make its own growth hormone, an injectable growth hormone restores growth. Growth hormone is administered to children who are not growing properly because they lack this hormone. If someone is borderline hypothyroid, this hormone may push him into hypothyroidism as it reduces the T4 hormone to abnormally low levels.

Drugs that remove thyroid hormone from your system

A number of drugs act upon the liver to speed up the metabolism of thyroid hormones into products that are not active. Other drugs pull thyroid hormones out of the body with bowel movements. These drugs are very common.

If you are taking a thyroid replacement hormone and you use one of these drugs, you may develop hypothyroidism. Ask your doctor to check your thyroid hormone levels about a month after you start taking one of these drugs: ߜ
Aluminium hydroxide:
Used to treat peptic ulcers.

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Carbamazepine and phenytoin:
Used to treat convulsions.

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Colestyramine and colestipol:
Used to reduce fats.

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Ferrous sulphate:
Given to people who are deficient in iron and have anaemia. At some point, the ferrous sulphate treatment is stopped when the person’s iron reserves are full. The person may actually become hyperthyroid at that point, if his thyroid hormone dose was increased due to the initial effects of the drug.

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Phenobarbital:
Used for the treatment of convulsions.

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Rifampicin:
One of the treatments for tuberculosis. This drug very rarely causes hypothyroidism in a person who takes thyroid hormone.

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Sucralfate:
Used for peptic ulcer disease. It may result in hypothyroidism if it’s taken long-term.

Importantly, you need to monitor your thyroid function both during and after use of a short-term medication that lowers the levels of thyroid hormone in your blood, especially if you are taking oral thyroid medication.

Drugs that decrease your TSH

The following drugs can lower the level of thyroid-stimulating hormone in your system, potentially leading to lower thyroid function: ߜ
Acetylsalicylic acid:
This drug is more commonly known as aspirin.

People taking more than 8 or 10 aspirin daily may suffer this effect on TSH.

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Bromocriptine:
Given to prevent lactation (milk production) and to shrink prolactin-secreting pituitary tumours.

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Dopamine:
Used to lower blood pressure, especially in an emergency setting. It lowers TSH but is not usually used long enough to cause problems with hypothyroidism.

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Corticosteroids:
Used to treat inflammation and reduce immunity.

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Octreotide:
A drug that treats certain tumours that produce hormones, especially acromegaly, which produces excessive growth hormone, and carcinoid tumours, which produce a chemical that causes severe diarrhoea and flushing of the skin.

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Opiates:
Drugs including morphine and heroin (diamorphine), which are used legally for pain control and illegally for a chemical high.

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Phentolamine:
Given to control blood pressure in someone with a tumour of the adrenal gland called a phaeochromocytoma. It reduces TSH but is not generally given long enough to make a difference in thyroid function.

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Pyridoxine:
Also called vitamin B6. This drug is taken for premenstrual syndrome, and when a person shows evidence of B6 vitamin deficiency.

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Thyroid hormones:
Given to replace a deficiency or to suppress thyroid cancer or a goitre. They suppress the production of TSH.

Creating false test results

Certain drugs can alter the thyroid hormone (T3 and T4) tests that measure total thyroid hormones (but not free thyroid hormones, which remain normal).

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If you’re taking one of the drugs listed here, keep that fact in mind if your total T4 test shows hypothyroidism but you aren’t experiencing any symptoms of the condition.

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Salicylates:
Aspirin is the most commonly used drug in this group.

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Phenytoin and carbamazepine:
Used as treatment for convulsions.

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Furosemide:
Causes the loss of excess water through the kidneys.

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Heparin:
Used to prevent blood clots. This drug doesn’t change your thyroid function, but if you’re given an injection of low molecular weight heparin, a measurement of free T4 taken within 10 hours of the injection is falsely elevated.

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Orphenadrine:
Used in a number of drug preparations for the relief of muscle spasms. It is not a muscle relaxant but may reduce pain.

Causing an increase in thyroid activity

The following drugs increase the production of TSH, which can result in hyperthyroidism:

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Amphetamine:
A controlled drug sometimes used inappropriately as a weight-loss agent.

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Cimetidine and ranitidine:
Used to reduce acid secretion to treat peptic ulcers. Both can raise the TSH, but studies don’t show a change in thyroid function with these drugs.

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Clomifene:
Brings on ovulation to promote pregnancy. It has effects on several of the hormones in the pituitary gland, including TSH.

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L-dopa inhibitors:
Drugs such as chlorpromazine and haloperidol, used in the management of psychotic disorders. They raise TSH, although people do not generally become hyperthyroid.

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Metoclopramide and domperidone:
Used to control nausea and vomiting, especially after surgery.

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Iodine:
Raises TSH levels as it blocks the release of thyroid hormones.

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Lithium:
Raises TSH, in addition to all its other effects on thyroid function. In very rare cases, it can cause hyperthyroidism. (It causes hypothyroidism much more frequently.)