The full content of Annals is available to subscribers

Subscribe/Learn More  >
Articles |

The Free Triiodothyronine (T3) Index

[+] Article, Author, and Disclosure Information

Grant support: by the Medical Research Service of the Veterans Administration and National Institutes of Health General Research Support Grant No. 5 SO1 RR 05587-07.

▸Requests for reprints should be addressed to Clark T. Sawin, M.D.; Boston Veterans Administration Hospital, 150 South Huntington Avenue; Boston, MA 02130.

Boston, Massachusetts

© 1978 American College of PhysiciansAmerican College of Physicians

Ann Intern Med. 1978;88(4):474-477. doi:10.7326/0003-4819-88-4-474
Text Size: A A A

In a large number of normal men (n = 111) and women (n = 110) the free triiodothyronine (T3), index, calculated from serum total T3 and T3-uptake, was highly correlated with free T3, measured by equilibrium dialysis. The correlation was almost as high as that of the free thyroxine (T4) index with free T4. The correlations of the total T3, free T3, and free T3 index with, respectively, the total T4, free T4, and free T4 index were much lower, though still statistically significant. The free T3 index is clinically useful because serum total T3 may sometimes be misleading. Hyperthyroid patients with apparent T4-toxicosis and normal total T3 may have an elevated free T3 index and thus physiologically elevated levels of both thyroid hormones. Calculation of the free T3 index might also make possible the diagnosis of T3-toxicosis in a patient with a normal free T4 index and normal total T3. Total T3 may be elevated without an elevated total T4 in women taking oral contraceptives; thus the free T3 index may prevent a misdiagnosis of T3-toxicosis. The free T3 index seems no better than total T3 in the diagnosis of primary hypothyroidism, but it can confirm the diagnosis of T3-hypothyroidism.





Citing articles are presented as examples only. In non-demo SCM6 implementation, integration with CrossRef’s "Cited By" API will populate this tab (http://www.crossref.org/citedby.html).


Submit a Comment/Letter
Submit a Comment/Letter

Summary for Patients

Clinical Slide Sets

Terms of Use

The In the Clinic® slide sets are owned and copyrighted by the American College of Physicians (ACP). All text, graphics, trademarks, and other intellectual property incorporated into the slide sets remain the sole and exclusive property of the ACP. The slide sets may be used only by the person who downloads or purchases them and only for the purpose of presenting them during not-for-profit educational activities. Users may incorporate the entire slide set or selected individual slides into their own teaching presentations but may not alter the content of the slides in any way or remove the ACP copyright notice. Users may make print copies for use as hand-outs for the audience the user is personally addressing but may not otherwise reproduce or distribute the slides by any means or media, including but not limited to sending them as e-mail attachments, posting them on Internet or Intranet sites, publishing them in meeting proceedings, or making them available for sale or distribution in any unauthorized form, without the express written permission of the ACP. Unauthorized use of the In the Clinic slide sets will constitute copyright infringement.


Buy Now for $42.00

to gain full access to the content and tools.

Want to Subscribe?

Learn more about subscription options

Topic Collections
PubMed Articles
Forgot your password?
Enter your username and email address. We'll send you a reminder to the email address on record.