The Butterfly Effect: A Small Gland, A World of Trouble

The thyroid, a butterfly-shaped gland in the throat, is responsible for maintaining warmth, providing energy, and protecting organ function.¹

The gland is responsible for the creation of thyroid hormone—90% inactive hormone (tetraiodothyronine; thyroxine; T4) and 10% active hormone (triiodothyronine; T3).² T4 must be converted to T3 to exert its effects; put simply, an iodine atom is removed from T4, transforming it to T3.^1,2 Thyroid hormone plays an essential role in almost every organ system in the body, and particularly in metabolism and protein synthesis.²

Thyroid diseases, including hypothyroidism and hyperthyroidism, are clinical states characterized by an underactive or overactive thyroid, respectively.^3,4 In the United States, 0.3% of adults have hypothyroidism and approximately 1% to 1.3% have hyperthyroidism.^3,5

Hypothyroidism

Hypothyroidism can be classified as primary or secondary hypothyroidism. Primary hypothyroidism is caused by a deficiency in thyroid hormone and low serum free T4 (FT4).³ Secondary hypothyroidism is a deficiency in thyroid-stimulating hormone (TSH; secretion of TSH stimulates the production of T3 and T4).^3,6

Therapy of choice for primary and secondary hypothyroidism is levothyroxine (Synthroid; AbbVie), a synthetic T4 hormone.⁷ The American Association of Clinical Endocrinologists and American Thyroid Association (ATA) recommend initiating adults on 1.6 mcg/kg/day of levothyroxine and dose adjusting every 4 to 6 weeks.^8,9 Dose adjustments (typically increasing or decreasing by 12.5-25 mcg/day) are based on thyroid hormone and TSH serum levels.⁸ Adjustments continue until the patient-specific TSH goal is reached.⁸ Levothyroxine tablets are available in a variety of dosage strengths (and corresponding colors; see Table 1⁷), allowing prescribers to titrate a patient’s dose carefully.

Treatment of primary hypothyroidism with levothyroxine has 3 goals⁸:

• Resolution of patient’s signs, symptoms, and laboratory and physiologic markers;
• Normalization of TSH and thyroid hormone concentrations; and
• Prevention of overtreatment.

In secondary hypothyroidism, the goal of levothyroxine treatment is to maintain a FT4 serum level within the upper half of the reference range.⁸

Several factors, including meals, medications, and supplements, may impair the absorption and metabolism of levothyroxine products.⁸ For optimal absorption, the ATA recommends the patients separate their levothyroxine dose from meals. It should be taken 60 minutes before breakfast or 3 hours after dinner. Additionally, patients should be advised to separate their levothyroxine dose from interfering medication or supplements (ie, calcium carbonate, ferrous sulfate) by at least 4 hours.^7,8

Hyperthyroidism

Thyrotoxicosis is a condition characterized by high production and secretion of thyroid hormone; hyperthyroidism is a form of thyrotoxicosis.⁴ Hyperthyroidism is characterized as either overt or subclinical based on serum levels of TSH, T3, and FT4 (see Table 2⁴).

Therapy of choice for hyperthyroidism is based on the cause. One of the most common causes is Graves disease (GD).⁴ Table 3^4,10 describes ATA-recommended treatment choices for GD.

Patients treated with radioactive iodine (RAI) and anti-thyroid drugs (ATDs) must receive a pregnancy test 48 hours before treatment or after a light menstrual period.⁴ Additionally, the following monitoring parameters are required after initiation of therapy⁴:

• RAI: serum FT4, total T3, and TSH must be checked within 1 to 2 months, then 4 to 6 weeks thereafter
• ATD: serum FT4 and total T3 must be checked within 2 to 6 weeks
• Thyroidectomy: TSH must be checked within 6 to 8 weeks

Conclusion

Hypothyroidism and hyperthyroidism are common endocrine disorders impacting millions of Americans. Levothyroxine is the therapy of choice for hypothyroidism, while hyperthyroidism is treated based on the cause—for GD, patients can be treated with RAI, ATDs, or thyroidectomy. A knowledgeable pharmacy team can ensure proper management and positively impact a patient’s well-being.

REFERENCES

1. Thyroid function tests. American Thyroid Association. Accessed November 12, 2025. https://www.thyroid.org/thyroid-function-tests/

2. Armstrong M, Asuka E, Fingeret A. Physiology, thyroid function. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Accessed November 12, 2025. https://www.ncbi.nlm.nih.gov/books/NBK537039/

3. Chaker L, Bioanco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550-1562. doi:10.1016/S0140-6736(17)30703-1

4. Ross RD, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343-1421. doi:10.1089/thy.2016.0229

5. Taylor PN, Albrecht D, Scholz A, et al. Global epidemiology of hyperthyroidism and hypothyroidism. Nat Rev Endocrinol. 2018;14(5):301-316. doi:10.1038/nrendo.2018.18

6. Shahid MA, Ashraf MA, Sharma S. Physiology, thyroid hormone. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025. Accessed November 12, 2025. https://www.ncbi.nlm.nih.gov/books/NBK500006/

7. Synthroid. Package insert. AbbVie Inc; updated August 2022. Accessed November 12, 2025. https://www.accessdata.fda.gov/drugsatfda_docs/label/2022/021402s036lbl.pdf

8. Jonklass J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670-1751. doi:10.1089/thy.2014.0028

9. Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012;18(6):988-1028. doi:10.4158/EP12280.GL

10. Methimazole. Package insert. PAR Pharmaceutical Companies Inc; updated January 2012. Accessed November 12, 2025. https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/040350s016lbl.pdf