Thyroid Function Testing Basics

• Thyroid stimulating hormone (TSH) alone is usually an adequate screening test for outpatients (unless you suspect pituitary disease). 
• Measurement of free hormones are preferred to total hormone levels because total thyroid hormone levels are affected by binding proteins. 
• If indicated for hospitalized patients, order TSH and free thyroxine (FT4).  
• Send free triiodothyronine (FT3) if you suspect T3 toxicosis and in the setting of low TSH and normal FT4. 

Thyroid Function Testing in Hospitalized Patients

• Do NOT routinely order thyroid testing if:
  • Low pretest probability of thyroid disease
  • Patient has been on stable outpatient dose of thyroid replacement therapy
• Consider testing for thyroid disease if:
  • High pretest probability of thyroid disease
  • Clinical conditions in which addressing thyroid dysfunction could provide a treatment: unexplained tachyarrhythmia, delirium, heart failure
  • Medications associated with thyroid toxicity: amiodarone, high iodine consumption, lithium, MTOR inhibitors, check point inhibitors
• Bottom line: only send thyroid tests if you actually suspect thyroid disease. Otherwise, results may be difficult to interpret unless highly abnormal, given sick euthyroid syndrome is common in hospitalized patients (see Non-Thyroidal Illness Syndrome)

Premawardhana L. Thyroid testing in acutely ill patients may be an expensive distraction. Biochem Med (Zagreb). 2017;27(300):300-307.

Taylor Wootton, MD, FHM, Ruth Bates, MD, Things We Do for No Reason™: Routine Thyroid-Stimulating Hormone Testing in the Hospital. J. Hosp. Med; doi:10.12788/jhm.3347