Thyroid Medications

What are Thyroid Problems?

The thyroid gland is located on the front part of the neck below the thyroid cartilage (Adam's apple). This gland produces thyroid hormones, which regulate metabolism. Thyroid hormones regulate the body's energy, use of other hormones and vitamins, and the growth and maturation of body tissues.

Diseases of the thyroid gland include:

Thyroid Medications

Many thyroid medications can be used to treat the various thyroid disorders.This article will review the following therapies:

  • Thyroid hormone replacement to treat hypothyroidism (not enough thyroid hormone) or thyroid cancer
  • Medicines to treat hyperthyroidism (too much thyroid hormone)
  • Other treatments for hyperthyroidism
  • Radioactive iodine treatment

Thyroid Hormone Replacement

There is one purpose for taking thyroid hormone: to replace the thyroid hormone levels when your thyroid is underactive (hypothyroidism).

L-thyroxine (also called LT4 or levothyroxine)

L-thyroxine is the most commonly used form of thyroid hormone replacement.

  • This medicine contains the synthetic form of a thyroid hormone
  • L-thyroxine as medication is identical to the L-thyroxine which is the major hormone made by the thyroid gland
  • L-thyroxine is the most commonly prescribed form of thyroid hormone replacement

Initial L-thyroxine dose

Physicians should be aware of the following conditions that their patients may also have when prescribing L-thyroxine:

L-thyroxine is taken once each day by mouth

  • Stable blood levels of thyroid hormone are achieved when L-thyroxine is taken at approximately the same time each day, ideally the first thing in the morning on an empty stomach.
  • Avoid taking food within 1 hour and avoid calcium, iron sucralfate, aluminum-containing antacid, and multivitamins within 2 hours before or after the dose.
  • If a dose is skipped, two doses may be taken the following day.

Blood levels of thyroid stimulating hormone (TSH) should be checked approximately 4-6 weeks following each adjustment of LT4 dose.

  • L-thyroxine comes in oral forms, for patients to use at home, and intravenous forms used in a hospital-setting.

L-thyroxine during pregnancy and breastfeeding:

  • It is safe to take L-thyroxine during pregnancy.
  • The dose of L-thyroxine often must be increased during the first 20 weeks of pregnancy. TSH testing is recommended immediately after pregnancy is confirmed.

Side effects that your physician should be aware of when taking thyroid hormone replacement include:

Drug Interactions with L-thyroxine

L-thyroxine and other medications

Many other medications can affect the way L-thyroxine is absorbed from the gut. Notify your physician if you take L-thyroxine and any of the following medications:

Other Thyroid Hormone Replacements

L-triiodothyronine

  • Also known as T3 [(liothyronine (Cytomel)]
    • T3 has a very short half life and must be given two to three times each day
    • T3 is responsible for most of the functions of thyroid hormone. Most of T3 comes from the conversion T4 (either from the thyroid gland or from L-thyroxine administration) to T3
    • However, T3 is seldom used alone for the treatment of hypothyroidism
    • T3 is rarely used in combination with LT4 for the treatment of hypothyroidism, but carefully controlled studies in the United States have not shown any benefit from this combined therapy
    • T3 is used for short term therapy during the preparation of patients with thyroid cancer for diagnostic imaging or radioactive iodine treatment
  • Desiccated animal (pig) "natural" thyroid extract [(thyroid (desiccated) (Armour Thyroid, Westhroid)]
    • Contains both T3 and T4, though not in the same proportions as the human thyroid gland; this extract contains excess T3 compared to the amount of T4 for human physiology
    • The proportions of T3 and T4 may vary from bottle to bottle of Armour thyroid
    • Extract should not be prescribed for the treatment of hypothyroidism
    • There is no current practical use for extract in thyroid care

Anti-thyroid Medications

These medicines are used to treat an overactive thyroid gland by decreasing the output of thyroid hormone from the thyroid.

Propylthiouracil (also called PTU)

  • PTU blocks the production of thyroid hormone inside the thyroid gland
  • PTU is usually given in divided doses, 2 to 3 times a day
  • Blood levels of thyroid hormone do not drop until the stores of thyroid hormone in the thyroid are depleted, typically after 2-4 weeks
  • Thyroid hormone levels may take 1 to 4 months to normalize
  • Mild side effects include rash, low grade fevers, and itchiness
  • Rare but serious side effects include a reduction in white blood cells, which can prevent the body from fighting off an infection, and hepatitis (liver damage).
  • Recent studies have shown there is more liver toxicity with PTU and so now, Methimazole is usually the drug of choice. PTU is still preferred by some in pregnancy even though there are more minor birth defects seen with it.

Methimazole (Tapazole)

  • Similar mechanism and delay of action as PTU
  • Tapazole can be taken once each day
  • Tapazole has similar side effects to PTU

Blood levels of thyroid hormones should be checked every 4 to 6 weeks until hormone levels stabilize

Other Medications for Hyperthyroidism

Beta-blockers (propranolol, Inderal, Inderal LA, InnoPran XL)

  • This medicine can help block the body's reaction to hyperthyroidism
  • It can control heart-related symptoms, such as palpitations, tremors, andagitation

Iodide solutions

(Lugol's solution, Strong iodine, super-saturated potassium iodide or SSKI)

  • Usually given as drops of liquid medicine, iodide solutions can prevent the release of hormone from an overactive thyroid gland.
  • Given only to treat Graves' hyperthyroidism
  • Must be given with an antithyroid medicine
  • May cause short term decrease in thyroid hormone levels, so often used prior to thyroid surgery
  • Common side effects are a metallic taste and nausea

Radioactive Iodine

131-iodine (131I or I-131) is the radioactive isotope used to kill both normal and cancerous thyroid cells

  • This is not to be confused with I-123, a harmless isotope used by radiologists in imaging and to determine thyroid activity
  • Radioactive iodine is safe to use in people who have allergic reactions with other iodine-containing compounds, including seafood and intravenous (IV) contrast media
  • The amount of elemental iodine in radioactive iodine is smaller than that in our daily diet

The three major uses for I-131 therapy

  1. hypothyroidism
  2. thyroid cancer
  3. toxic multinodular goiter

1) Hyperthyroidism

  • I-131 can be given to patients who have overactive thyroid tissue or enlarged thyroid glands to destroy normal thyroid tissue
  • Typically, the I-131 takes several months to have its full effect for eliminating the thyroid tissue

2) Thyroid cancer

  • Larger doses of I-131 are used to kill thyroid cancer cells

3) Nontoxic multinodular goiter

  • Used in Europe to shrink multinodular goiters by about 40%, but not typically used in the United States

Side effects are rare and easily treatable and include nausea, and pain or swelling of the thyroid tissue and salivary glands

Safety of radioactive iodine I-131

  • I-131 should never be used in women who are pregnant;
  • Pregnancy should be deferred by 6 months after I-131 therapy;
  • Long-term fertility is not affected in either men or women;
  • For lower doses of I-131 used for hyperthyroidism, the Nuclear Regulation Commission does not require isolation; still, general exposure to children and pregnant women should be avoided for 3 days after the treatment;
  • For larger doses of I-131 used in thyroid cancer patients, exposure to children and pregnant women should be avoided for 3 to 7 days after the treatment; and
  • Exposure of others to the body fluids (saliva, urine, nasal secretions, etc.) of a patient who has been recently treated with I-131 should be limited or avoided completely.

Medically reviewed by John A. Seibel, MD; Board Certified Internal Medicine with a subspecialty in Endocrinology & Metabolism

REFERENCE:

"Thyroid hormone action"
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