Triiodothyronine

Abstract

After 20 years in the wilderness, 3,5,3′‐triiodo‐l‐thyronine (T3) has come back home, for it is exactly 20 years since Gross & Pitt‐Rivers (1952) first drew attention to its presence in human serum. A few years later Pitt‐Rivers et al. (1955) demonstrated its formation from thyroxine (T4) given to patients with athyreotic myxoedema, and in 1957 Maclagen et al. reported thyrotoxicosis with elevated plasma concentration of T3 and normal concentration of T4‐the first identifiable instance of what is now referred to as ‘T3 toxicosis’. Despite these early suggestions of significant physiological and pathological contributions, T3 was regarded as a minor and unimportant iodinated compound until the development of an improved biochemical assay (Sterling et al., 1969) paved the way for wider recognition of its role in health and disease and stimulated the development of a sensitive and dependable radioimmunoassay. In the past few years information about the action, metabolism and clinical significance of T3 has rapidly accrued; this review attempts to put together some of this new and exciting information. Because methods for the assay of T3 in biological fluids have only recently been introduced, the validity of the various techniques will be considered first.

Simultaneous Radioimmunoassay of Thyroid Hormones in Unextracted Serum

“…Over the last few years the measurement of serum triiodothyronine (T3) has been introduced and is rapidly becoming accepted as a measurement yielding useful information. In particular it is thought to be useful in detecting early hyperthyroidism (Burke and Eastman, 1974), in establishing the diagnosis of T3 thyrotoxicosis (Sterling et al, 1970, Hollander, et 01., 1972, Shalet et 01., 1975, and in assessing thyroid status in patients treated for thyrotoxicosis both as an indicator of a euthyroid state in the presence of a low T4 level (Hoffenberg, 1973) and in providing early biochemical evidence of relapse (Marsden et 01., 1975). Many laboratories, including our own, now find themselves measuring serum T3 in an increasing proportion of samples sent for thyroid function tests, both for the specific reasons mentioned above and to clarify the position in any patient whose T4 result is equivocal.…”

Preparation of the intensive care patient for major surgery

“…Thyroid disease requires that a careful history and physical examination be obtained to establish a suspicion of either hyper-or hypothyroidism, as laboratory screening is not routine and many of the findings of both conditions occur commonly among ICU patients. Rapid treatment of hypothyroidism may require intravenous levothyroxine [14]. Hyperthyroid conditions are treated by blocking thyroid hormone synthesis with propylthiouracil.…”

The present role of in-vitro tests of thyroid function.

“…This is often explained by thyroidal secretion of hormone with a high T3/T4 ratio, so that serum T3 concentration is normal or high, the patient thus being maintained in a euthyroid state (Sterling, Brenner, Newman, Odell, and Bellabarba, 1971;Hoffenberg, 1973). Measurement of serum TSH and, perhaps, its response to TRH might throw further light on the problem, but as long as the patient remains well, fuller investigation or treatment would generally be unnecessary.…”