T4-to-T3 Conversion and Hypothyroidism: Medical Journals

Purpose of this compilation

To show that the T4-to-T3 conversion process often does not function in hypothyroid patients as it does in euthyroid patients. For these patients, T3 supplementation in addition to T4 is necessary to restore health.

See also the links in the right column as well as in our Desiccated Thyroid References and TSH References sections.

A. T4-to-T3 conversion process
B. Laboratory findings

A. T4-to-T3 conversion process

1. "Thyroid insufficiency. Is TSH the only diagnostic tool?" (Belgium, 2000)

The explanation is that TSH is grossly in feedback with serum T4 only, not so much with serum T3, while the patient's wellbeing depends on the free T3 that is disposable inside the cells. As hypothyroid patients are usually unable to convert inactive T4 into active T3, owing to a lack of 5' -deiodinase in the liver and kidneys, the administration of T4 can eventually correct the serum TSH level, but rarely provides the patient with the T3 needed to be relieved of his symptoms.

2. "T3 is at least as important as T4 in all hypothyroid patients" (US, 1993)

It is assumed that, except in the 'euthyroid sick syndrome' and certain special situations, such as lithium therapy (10. St Germain, 1987), T4 converts peripherally to T3 in fairly standard amounts and at fairly standard rates. It only takes the consistent measuring of both free-T3 and free-T4 bloodlevels, in all one's hypothyroid patients, every time, to very rapidly dispell this myth. If one believes that both the T3 and T4 hormones need to be in their mid- to high-normal ranges, one soon discovers that, while a certain percentage of hypothyroid patients do convert enough T4 to T3 at a sufficient rate for T4 treatment to be adequate as a source of T3, a substantial proportion of patients require some combination of both exogenous T3 and T4.

B. Laboratory findings

1. "Levothyroxine therapy and serum free thyroxine and free triiodothyronine concentrations" (US, 2002)

These findings indicate that in hypothyroid patients L-T4-replacement...that is sufficient to maintain a normal serum TSH, is accompanied by a serum free T4 that is higher than that in untreated euthyroid patients or normal individuals and may not result in an appropriately normal serum free T3 concentration.

2. "Effect of 3,5,3'L-triiodothyronine administration on serum thyroid hormone levels in hypothyroid patients maintained on constant doses of thyroxine" (Japan, 1980)

...3 patients with elevated TSH levels during T4 administration showed almost normal TSH levels after T4 and T3 ingestion. The results showed the reciprocal relationship between T3 and rT3 levels in serum after T3 administration in hypothyroid patients maintained on constant doses of T4. Furthermore, the present findings suggest that the administration of both T4 and T3 might be a more suitable replacement therapy in the patients with hypothyroidism than T4 alone.