THE THYROMIMETIC EFFECTS OF D-TRICHLOR-THYRONINE

Various aspects of thyroid activity of a non-iodine-containing thyroid analogue, 3:5:3′-trichloro-d-thyronine (d-trichlor-thyronine = d-ClT3) have been studied and compared with those of 3:5:3′-triiodo-l-thyronine (l-triiodo-thyronine = l-T3).

As judged by the suppression of radio-iodine uptake in normal subjects, d-ClT3 is about six thousand times less active than l-T3 as an inhibitor of TSH production. Acute loading tests in hypothyroid patients showed that, judged by the effect on b.m.r., d-ClT3 is over 1600 times less potent than l-T3. Maintenance studies in hypothyroid patients showed that d-ClT3 at a dosage of 90 mg./day for 1 month achieved only partial restoration of the euthyroid state, and failed to maintain it in patients previously receiving other replacement therapy; this is 1500 times the minimum maintenance dose of l-T3. It was, however, found that the blood cholesterol levels fell to, or remained within, normal limits in all cases, suggesting that, like many other d-isomers, d-ClT3 has a relatively high hypocholesterolaemic potency.

There is no evidence that d-trichlor-thyronine possesses greater pituitary-suppressive than calorigenic potency; it is therefore unlikely to be of value in the treatment of Graves' disease.

EFFECT OF SOME THYROXINE ANALOGUES ON HYPOTHYROID TADPOLES

Using a new tadpole assay for thyroid hormone which has previously been described, the activities of some thyroxine analogues have been investigated. Apart from the usual immersion technique, microinjection was also used for administration of drugs.

Diiodotyrosine was found to have no effect. Diiodothyronine only had a small effect. This compound as well as triprop, triac and tetrac gave dose response curves quite unlike those of triiodothyronine and thyroxine, so that an estimation of the relative activity could hardly be made. The implications of this finding is discussed.

D-thyroxine was found to be 7 times less active than L-thyroxine.

Triiodothyronine which in previous immersion experiments was found to be about 9 times as active as thyroxine was in injection experiments about 5 times as active as thyroxine.

None of the fatty acid derivatives investigated was more active than thyroxine if given by injection. Contamination with small amounts of these derivatives in the test material therefore does not invalidate the results obtained by the present method.