In The Thyroid Gland

The Natural Thyroid Diet

The Natural Thyroid Diet

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Fig. 5.8 Biosynthesis of the thyroid hormones. Thyroxine (T4) and triiodothyronine (T3) are synthesised in the thyroid gland from tyrosine residues in the protein thyroglobulin. The conversion of T4 to T3, the active hormone, occurs mainly in peripheral tissues.

Triiodothyronine (T3) no-

Fig. 5.8 Biosynthesis of the thyroid hormones. Thyroxine (T4) and triiodothyronine (T3) are synthesised in the thyroid gland from tyrosine residues in the protein thyroglobulin. The conversion of T4 to T3, the active hormone, occurs mainly in peripheral tissues.

they are formed from tyrosine residues within thyroglobulin, and iodine which is taken up avidly by the gland from the blood (Fig. 5.8). There are two thyroid hormones, known as thyroxine or T4 (with four iodine atoms per molecule) and triiodothyronine or T3 (with three iodine atoms). Both are secreted by the gland and present in blood, although it appears that T3 is the active hormone. Most tissues express the enzyme necessary to convert T4 to T3.

Synthesis and secretion of the thyroid hormones are regulated by the pituitary-derived thyroid-stimulating hormone (TSH: see Section 5.3.1). TSH also increases thyroid size. In thyroid deficiency due, for instance, to lack of iodine in the diet, thyroid hormone levels in the blood are low. As discussed above (Section 5.3.1), this leads to an increase in TSH secretion in order to stimulate more thyroid hormone production. It also leads to enlargement of the thyroid gland, sometimes to a massive growth on the neck known as a goitre; hence the apparent paradox of an enlarged gland and a deficient hormone.

Most of the hormones which regulate metabolism do so in a very rapid manner; their secretion is regulated on a minute-to-minute basis and their effects on metabolic pathways are similarly rapid, or sometimes somewhat slower if effects on protein synthesis are involved. The thyroid hormones, however, seem to set the general level of metabolism in a long-term way. In parallel with this, the thyroid gland is unusual in storing a large amount of hormone - enough for around three months' secretion.

Some specific effects of thyroid hormones on metabolism will be covered in later chapters, particularly their effect on muscle protein metabolism (Section 6.3.3). For the most part, however, the thyroid hormones play a 'modulating' role, affecting the level of response to other hormones. In particular, they regulate the sensitivity of metabolic processes to catecholamines (adrenaline and noradrenaline): thus, an excess of thyroid hormones has many similarities to an excess of adrenaline or noradrenaline. An excess of thyroid hormones is characterised by an increase in the overall metabolic rate; a deficiency is characterised by a depression of metabolic rate.

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