In screening for primary hypothyroidism, only serum thyroid-stimulating hormone is required as a first-line test1–3
In the general adult population (excluding pregnant women and older people), a normal thyroid-stimulating hormone (TSH) level is defined as the 95% laboratory-specific reference interval (about 0.45–4.50 mIU/L).1–3 In adults (other than in pregnancy), TSH values greater than 10 mIU/L or TSH elevations with low free thyroxine values are generally considered indications for levothyroxine treatment1–4 (Appendix 1, available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.141596/-/DC1).
Maintenance of a normal serum TSH level is the mainstay of biochemical follow-up of primary hypothyroidism
In general, maintenance of a normal TSH level (about 0.45–4.50 mIU/L) is the biochemical goal in primary hypothyroidism (Box 1);5 age-specific target TSH levels have been recommended.1–4 Higher upper limits of TSH targets are acceptable for older people (e.g., up to 6 mIU/L in patients aged > 65 yr, with consideration of comorbidities).2,3
Box 1: Choosing Wisely Canada recommendation on hypothyroidism.
Don’t use free thyroxine or triiodothyronine to screen for hypothyroidism, or to monitor and adjust levothyroxine (thyroxine) dose in patients with known primary hypothyroidism.5
About a third of patients receiving treatment for hypothyroidism have TSH values outside the target range1–3
Annual TSH monitoring, with more frequent monitoring in special circumstances (e.g., pregnancy, major weight change or addition of potentially interacting medications), may facilitate appropriate dose adjustment. 3,4 If needed, the levothyroxine dose may be with changes of about 12.5–25 μg, measuring TSH about four to eight weeks later.1,4
Coingestion of levothyroxine with food may cause impaired absorption and should be avoided1,3
Ideally, levothyroxine should be taken only with water at a consistent time, either one hour before breakfast or at bedtime more than three hours after the final meal of the day.1,3 If an alternative schedule is chosen, it should be consistently maintained.
Coingestion of levothyroxine with medications or dietary supplements that may interfere with its absorption should be avoided2,3
Some medications ( e.g., bile acid sequestrants, phosphate binders, aluminum-containing antacids) and dietary supplements (e.g., calcium, iron) may interfere with levothyroxine absorption. Ideally, a four-hour separation from taking levothyroxine is advised.2,3 Lists of drugs that interfere are available.1,3
CMAJ is collaborating with Choosing Wisely Canada (www.choosingwiselycanada.org), with support from Health Canada, to publish a series of articles describing how to apply the Choosing Wisely Canada recommendations in clinical practice.
Acknowledgements
The authors thank Dr. Shereen Ezzat, of the University Health Network and the University of Toronto, for reviewing the manuscript.
Footnotes
Competing interests: Anna Sawka is a member and Jacqueline Jonklaas is chair of the American Thyroid Association Thyroid Replacement Task Force (unpaid). Jacqueline Jonklaas holds an R01 research grant from the National Institutes of Health for hypothyroidism treatment in aging and thyroid cancer.
This article has been peer reviewed.
References
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