Iodothyronines and iodotyrosines as hypothetical receptors for catecholamines and opiates

Di-iodothyronine as part of the oestradiol and catechol oestrogen receptor--the role of iodine, thyroid hormones and melatonin in the aetiology of breast cancer

Hypothyroidism and low iodine intake may be important aetiological factors in oestrogen dependent tumours of the breast, uterus and ovary. They are preventable risk factors. Iodine supplementation will hopefully lead to a decreased incidence of these cancers in future generations. The present author proposes that the tyrosyl residue in the hydrophobic oestrogen binding site of the oestrogen receptor is post translationally modified to monoiodotyrosine and hence 3,3' di-iodothyronine monoamine (T2) by peroxidase activity. He has previously proposed that various monoamine receptors are also T2 based. The densities of these receptors are increased in hypothyroidism and they exert control over release of prolactin and other hormones, including melatonin at multiple sites in the hypothalamic--pituitary axis. Melatonin is a metabolite of serotonin and hence melatonin receptors may be T2 or rT3 based as well. These factors could be significant in the aetiology of breast cancer as high prolactin and melatonin levels may be protective. Oestrogen receptor density may be increased in hypothyroidism as is certain monoamine receptor density. This would amplify the effect of high circulation oestrogen levels in hypothyroidism and may help explain why hypothyroidism and low iodine intake are risk factors for breast, uterine and ovarian cancer.

Antidepressant-like effects of triiodothyroacetic acid in mice

Triiodothyroacetic acid (TA3) is a natural thyroid analogue which possesses some of the properties of triiodothyronine (T3). In particular, it has an inhibitory effect on the thyreotropic axis, but its peripheral effects are reduced. Given the activity of T3 on psychopharmacological models of depression in rodents, we investigated the effects of TA3 on some pharmacological and behavioral tests in mice. TA3 antagonized apomorphine- and oxotremorine-induced hypothermia, potentiated yohimbine-induced toxicity and L-5-hydroxy-tryptophan-induced head twitches, but did not affect forced swimming-induced immobility or reserpine-induced hypothermia. Thus, TA3 was effective on the same psychopharmacological tests which have previously been used to demonstrate the antidepressant-like effects of T3. Moreover, TA3 under the same experimental conditions as T3 has little effect on the metabolic clearance rate parameters, and might, therefore, be preferable to T3 for clinical use in depression.

Reverse tri-iodothyronine as part of alpha 2 adrenergic receptors

Thyroid hormones affect alpha and beta adrenergic receptor number profoundly. Presynaptic and post synaptic alpha 2 adrenergic receptor number is decreased in hypothyroidism. Behavioural studies show decreased presynaptic alpha 2 adrenergic function in hypothyroidism and increased function in hyperthyroidism. Reverse tri-iodothyronine (rT3) levels are also low in hypothyroidism and levels are high in hyperthyroidism induced by T4 administration. The present author has proposed that rT3 is incorporated into the alpha 2 adrenergic receptor where it provides the aromatic binding site. Tri-iodothyronine (T3) administration does not raise alpha 2 adrenergic receptor numbers above normal control values. This is consistent with the author's hypothesis as T3 cannot be converted to rT3. This paper describes how rT3 could provide the aromatic binding site for drugs that act on alpha 2 adrenergic receptors.