Thyroid Medications

There are many thyroid medications. The following are the main ones in New Zealand. 

Levothyroxine or T4 is thyroid hormone. Levothyroxine is commonly called thyroxine. Tablets come in 50mcg and 100mcg sizes.

In New Zealand there are four brands of levothyroxine:

1. Eltroxin,
2. Synthyroid,
3. Goldshield levothyroxine
4. and Eutroxsig

Levothyroxine is taken once a day 30-60 minutes before breakfast. Thyroid blood tests are done 4-6 weeks after commencing thyroxine to check that you are getting the right dose for your body. The blood test checks two hormones: TSH (thyroid stimulating hormone) and fT4 (free T4).

For replacement therapy, both the TSH and fT4 should be within the normal range. Hypothyroid patients with heart disease and the elderly should start thyroid hormone replacement therapy at a low dose and the dose should be gradually increased until the TSH and fT4 are normal.  

For patients with thyroid cancer, the levothyroxine dose depends on the risk stratification. Patients are divided into three groups: low, medium and high risk.

1. For low risk patients the TSH level is between 0.5-2.0 mU/L,
2. for intermediate risk patients the TSH level is between 0.1-0.5 mU/L
3. and for high risk patients the TSH level is <0.1 mU/L.

When the dose of thyroxine is designed to reduce the TSH below the normal range, it is called a suppressive dose of thyroxine. Please note the normal TSH level is 0.27-4.2 mU/L.  

Thyroxine is given at the dose of 1.6mcg per kg and adjusted until the TSH is normal or in the targeted range for patients with papillary or follicular thyroid cancer. There are various online calculators that will estimate the dose of thyroxine depending on the patient’s age, gender, height, weight and target level of TSH.

Because it takes 4-6 weeks after a thyroxine dose adjustment for the new thyroid levels to settle, a blood test is done 4-6 weeks after any dose adjustment.  

Finally, if you miss a dose of thyroxine, take it as soon as you remember. You can even take two doses at once the following day, provided you are not elderly, and you don’t have heart disease.  

Triiodothyronine or T3 is the other main thyroid hormone. Cytomel is the trade name for liothyronine (triiodothyronine). T4 is converted to T3 by the enzyme deiodinase (DIO). Most people who are hypothyroid take only levothyroxine. There is considerable debate in the medical literature about whether there is any advantage taking T3 as well as thyroxine (T4). It is thought that some people have abnormalities in their deiodinase genes (DIO1/ DIO2) and that these people may benefit from combined T4 and T3 therapy.  The recommended ratio of T4 to T3 is approximately 15 to 1. If T3 is added, the T4 dose is reduced.

It is also possible to take just T3 instead of thyroxine. In healthy adults the usual starting dose of T3 is 25mcg per day and is adjusted up to 75mcg per day depending on the measured TSH level. Note that 25mcg of T3 is roughly equivalent to 100mcg of T4.  Because T3 works faster (shorter half-life) than thyroxine, it may be easier on patients to start and stop T3. This is sometimes done when patients are going to have radioactive iodine.  

This is obtained from pigs and contains both T4 and T3. The dose of T3 and T4 in whole thyroid extract is not standardised, so it may be difficult to get good control of hypothyroidism as judged by blood tests (TSH, fT4 and fT3). Dr Hall has no experience with whole thyroid extract.  

Carbimazole is the most common medication to treat hyperthyroidism. The initial starting dose depends on the degree of hyperthyroidism. Mild, moderate and severe hyperthyroidism is treated with 20, 40 and 60 mg daily respectively divided into 2-3 doses. Once control of hyperthyroidism is achieved, a maintenance dose of 10-15mg per day is maintained for 1-2 years. Carbimazole decreases the production of thyroid hormone by inhibiting the enzyme thyroid peroxidase which is involved in the oxidising, organification and coupling steps of thyroid hormone production.

Because the thyroid gland stores thyroid hormones, carbimazole takes 2-4 weeks for any decrease in the production of thyroid hormone to result in a reduction of thyroid hormone levels in the blood.  Because carbimazole can cause dramatic changes in the number of red blood cells, white blood cells and platelets, and can cause liver damage, blood tests (full blood count and liver function tests) are done at the start of treatment.

Patients are advised to report immediately any fever, mouth ulcers or sore throat so that urgent repeat full blood count  and liver function blood tests can be performed. Carbimazole is stopped immediately if there are signs of bone marrow or liver damage. Carbimazole is contraindicated in the first trimester of pregnancy. Over 50% of patients develop recurrent hyperthyroidism after stopping carbimazole.

Propylthiouracil is used to treat hyperthyroidism in patients for whom carbimazole is not suitable. PTU inhibits the enzyme thyroid peroxidase which is involved in the oxidising, organification and coupling steps of thyroid hormone production. PTU also inhibits the enzyme 5’-deiodinase which is involved in converting T4 to T3. T3 is the more active thyroid hormone and has greater metabolic activity.

Long term use can cause liver damage and bone marrow damage. The usual starting dose is 100 mg three times a day. The maintenance dose is 50-150 mg daily. PTU works within 24-36 hours. PTU is contraindicated in pregnancy. 

Thyrogen is recombinant thyroid stimulating hormone (TSH). It is used to increase TSH for patients undergoing radioactive iodine. It is given as an injection over two days with RAI given on the third day. Thyrogen is used instead of stopping thyroxine for 4 weeks. It is expensive and is not funded in New Zealand.  

Calcium is given to patients who have a low calcium level following total or completion thyroidectomy. It is given three times a day and the dosage is reduced depending on changes in the calcium level measured with a blood test. Calcium carbonate contains 40% elemental calcium, while calcium citrate contains 21% elemental calcium. Calcium may cause constipation, gas and bloating. Calcium carbonate is more constipating than calcium citrate. Calcium carbonate is better absorbed when taken with food. Calcium citrate is better absorbed than calcium carbonate but contains less calcium than calcium carbonate. Calcium citrate may be better in the elderly and those taking medications for indigestion.  

Calcitriol is the active form of vitamin D. It is prescribed to patients who have a low calcium level following total or completion thyroidectomy. It is given twice a day and the dose reduced depending on the calcium level. It is well tolerated.  

Vitamin D is the sunshine vitamin. It is made in the skin under the influence of ultraviolet light. It is changed into its active form by the liver and the kidneys. The American Thyroid Association (ATA) recommends that individuals planning thyroid surgery have their vitamin D level checked. This is easy to do with a simple blood test. Studies have shown that correcting vitamin D levels prior to thyroid surgery lowers the risk of developing a low calcium level after surgery. Very low vitamin D levels are easily corrected by taking vitamin D at 1.25mg daily for ten days.   

Lugol’s iodine is used prior to surgery in patients with hyperthyroidism to reduce the blood loss and thereby make the surgery easier. It is thought to be of most use in patients with Grave’s disease. Six drops of Lugol’s iodine (8 mg iodide/drop) mixed with some water is taken three times a day for ten days prior to surgery.