Hypothyroidism is characterized by not having enough circulating thyroid hormone, a condition that affects up to 4.6% of the United States population. It is most commonly experienced by individuals that are over 60 years of age and biologically female.
Many different medicines are available to treat hypothyroidism. These medicines, by supplementing thyroid hormones, help the body regulate energy and metabolism. One of the primary focuses of treatment is correcting TSH (thyroid stimulating hormone) and T4 (thyroxine) levels throughout the body. TSH is a hormone that stimulates the thyroid gland to produce T4. T4 then becomes converted to T3 (triiodothyronine). T3 is the active form of thyroid hormone. Treatments are focused on supplementing T4 and/or T3.
A diagnosis of hypothyroidism focuses on having low T4 levels and elevated TSH levels. If there is enough T4 in the body, then a signal is produced for TSH production to slow down. If there’s not enough T4 in the body, TSH levels elevate because the body is attempting to produce more T4.
FAST FACT: for a detailed breakdown on the relationship between TSH, T4, and T3, this resource from the American Thyroid Association provides a summarized breakdown.
Compared to many other conditions, medicines for the treatment of hypothyroidism have more nuances and tidbits to be aware of. The list below highlights specific distinctions for hypothyroidism treatment.
Levothyroxine
Levothyroxine, a synthetic form of T4, is the preferred first-line treatment for hypothyroidism. It is available as a generic product and as multiple brand-name products. Brand names for levothyroxine are Synthroid, Levoxyl, Tirosint, and Unithroid.
Compared to most other medications where there is no notable difference between taking a generic or brand name product, there are reasons to be consistent with taking a specific formulation of levothyroxine. Maintaining a consistent formulation (i.e., a product from the same manufacturer and of the same formulation) is crucial to allow the body to retain consistent drug levels over time.
There are no noted differences in effectiveness between generic and brand name products. Rather, it’s important to stay consistent regarding which product is taken over time. This is a joint statement made by the American Thyroid Association, the Endocrine Society, and the American Association of Clinical Endocrinologists.
One benefit of administering T4 is that the body is able to naturally convert the proper ratio it needs to T3. T4 acts as a precursor to T3 (active thyroid hormone), and the body likes to retain a certain balance between T4 and T3. Levothyroxine products are the hypothyroid drugs of choice because they have a higher degree of chemical stability, have a lower cost, and are more uniform in potency compared to other hypothyroid medicines (listed below).
In general, one formulation of levothyroxine may be switched to another because of cost, inventory issues, or preferred product status changes on an insurance formulary. However, switching products (i.e., levothyroxine to Synthroid) can cause significant changes with how well the drug works or on how well side effects are tolerated. If a levothyroxine product must be changed, TSH levels should be re-evaluated by the prescribing physician via blood work. If necessary, the dose of levothyroxine should then be adjusted.
No matter the formulation, levothyroxine is measured in microgram (mcg) doses. It is available as a tablet, capsule, injection, and oral liquid solution.
FAST FACT: Levothyroxine is synthetic T4. No matter the manufacturer/brand of levothyroxine, tablet colors are consistent and correlate to tablet strength. Tablets are orange (25 mcg), white (50 mcg), violet (75 mcg), olive (88 mcg), yellow (100 mcg), rose (112 mcg), brown (125 mcg), turquoise (137 mcg), blue (150 mcg), lilac (175 mcg), pink (200 mcg), and green (300 mcg).
Liothyronine
Compared to levothyroxine (synthetic T4), liothyronine is synthetic T3. Liothyronine is available to be dispensed as a generic product and under the brand names of Cytomel and Triostat. Liothyronine (T3) is the active form of thyroid hormone.
Similarly to levothyroxine products, it is not advisable to switch between liothyronine products. If liothyronine is switched to Cytomel (or vice versa), it is important to follow up with the prescribing physician to have blood work obtained to assess drug levels.
Patients may be switched to liothyronine from levothyroxine for a number of reasons. One of the more significant reasons is in individuals that have a certain genetic defect called a deiodinase polymorphism. In order to convert T4 to T3, a certain enzyme is needed to make this conversion possible. In individuals with this defect, this conversion cannot be performed as this enzyme is missing, so no T3 is created. To bypass this, liothyronine or Cytomel could be given instead.
Regarding reasons aside from the one prior mentioned, they are typically patient specific. However, other common reasons are that an individual could be switched for simply not obtaining desired thyroid levels with levothyroxine or by having side effects with levothyroxine products. At the physician’s discretion, liothyronine could be added in addition to a levothyroxine or in replacement of a levothyroxine product.
As T3 is the active form of T4, it may seem like that should be the ideal treatment choice. However, like earlier stated, it’s typically better to administer T4 to allow the body to naturally synthesize the ratio of T3 that it needs instead of simply administering T3. Additionally, liothyronine is more rapidly absorbed in the intestines than levothyroxine so too much drug accumulation may happen too quickly, leading to concerns of overcorrection (hyperthyroidism).
No matter the formulation, liothyronine is measured in microgram (mcg) doses. It is available as both a tablet and injection.
FAST FACT: Liothyronine is synthetic T3.
Desiccated Thyroid, USP
Desiccated thyroid is sometimes referred to as natural thyroid. These products contain both T3 and T4. Desiccated thyroid is not dispensed as a generic product. Rather, it is dispensed under the brand names Armour Thyroid, Nature-Throid, NP Thyroid, Westhroid, and WP Thyroid.
For a large part of the 1900s, combination T3/T4 therapy with desiccated thyroid products were the mainstays of hypothyroidism therapy. However, in the 1970s, after solidifying that T3 is converted from T4, by gaining better lab tests to measure TSH levels, and with synthetic T3 and T4 products becoming available and affordable, standard practice measures were switched away from prescribing desiccated thyroid products as a first-line treatment option.
From a practice standpoint, desiccated thyroid products are prescribed lower in preference order than levothyroxine or liothyronine because of the possibility that these products can contain variable amounts of T3 and T4. Since these are naturally-derived products and not synthetic products, it’s harder to keep the same level of T3/T4 in each tablet from batch to batch. However, these products are still occasionally prescribed by physicians for historical reasons, if a patient has been stable on them for a prolonged period of time, or if a patient is having miscellaneous issues with levothyroxine-based and/or liothyronine-based therapy.
Of note, these products are currently pork-derived. Historically, they were also cow-derived. No matter the formulation, desiccated thyroid products are available as tablets. They are typically measured in grains (1 grain = 65 mg).
FAST FACT: Desiccated thyroid contains both T3 and T4, and it is considered to be a natural thyroid product. Desiccated thyroid products should be avoided in individuals who do not eat pork.
Liotrix
Liotrix was available to be dispensed under the brand name Thyrolar. Thyrolar contained both T3 and T4 in a 1:4 ratio. However, on December 6, 2018, Thyrolar was discontinued by the United States Food and Drug Administration (FDA). Because of this discontinuation, hypothyroidism treatment options consist of levothyroxine, liothyronine, and desiccated thyroid-based products.
Other Things to Remember While Taking Hypothyroid Medications
- It is recommended to take thyroid medicine products on an empty stomach, typically 60 minutes (or more) before breakfast.
- Antacids, calcium, iron, magnesium, multivitamins, cholestyramine, sevelamer, and sucralfate can reduce levothyroxine levels and should be separated by four hours from levothyroxine.
- SSRIs (antidepressants) and estrogen products can decrease thyroid hormone levels.
- For individuals taking warfarin, taking thyroid hormones can increase INR levels.
- For individuals taking medicines for osteoporosis, it is typically recommended to take thyroid medicines 30 minutes before medicines like Fosamax (alendronate).
- Thyroid hormone supplementation should never be used as a weight-loss mechanism.
- If you are experiencing cold sensitivity, dry skin, fatigue, weight gain, weakness, constipation, goiter, muscle cramps, etc., that could be a sign of hypothyroidism (current thyroid medicine dose may be insufficient).
- If you are experiencing increased heart rate, sweating, unexplained weight loss, or irritability, that could be a sign of hyperthyroidism (too high of a thyroid medicine dose).
- Blood work should be obtained at a minimum on an annual basis. This is to ensure that current doses are thyroid medication remain adequate over time.
Resources:
- Benvenga S, Carlé A. Levothyroxine Formulations: Pharmacological and Clinical Implications of Generic Substitution. Adv Ther. 2019;36(Suppl 2):59–71. doi:10.1007/s12325-019-01079-1
- American Thyroid Association. Hypothyroidism FAQs. Accessed 19 April 2020.
- Celi, F.; Zemskova, M.; Lindemann, J. et al. Metabolic effects of liothyronine therapy in hypothyroidism: a randomized, double-blind, crossover trial of liothyronine versus levothyroxine. J Clin Endocrinol Metab. 2011 Nov;96(11:3466-74. doi:10.1210/jc.2011-1329.
- McAninch EA, Bianco AC. The Swinging Pendulum in Treatment for Hypothyroidism: From (and Toward?) Combination Therapy. Front Endocrinol (Lausanne). 2019;10:446. Published 2019 Jul 9. doi:10.3389/fendo.2019.00446
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