Thyroid gland volume

Introduction of iodine fortification in an iodine-deficient population results in lowering thyroid gland volumes. 

Propylthiouracil is rapidly absorbed, reaching peak serum levels after 1 hour. The bioavailability of 50-80% may be due to incomplete absorption or a large first-pass effect in the liver. The volume of distribution approximates total body water with accumulation in the thyroid gland. Most of an ingested dose of propylthiouracil is excreted by the kidney as the inactive glucuronide within 24 hours. 

In contrast, methimazole is completely absorbed but at variable rates. It is readily accumulated by the thyroid gland and has a volume of distribution similar to that of propylthiouracil. Excretion is slower than with propylthiouracil 65-70% of a dose is recovered in the urine in 48 hours. 

Lithium affects thyroid function (52-56), and in most patients, after 4 months of treatment, there is a transient fall in serum levels of thyroxine (T4) and a rise in thyrotropic hormone (thyroid-stimulating hormone, TSH). After 1 year of treatment, these hormones have generally returned to their baseline. The mechanisms for this are obscure, but lithium inhibits both thyroxine synthesis and its release from the gland (201). Lithium may inhibit endocytosis in the thyroid gland, which results in an accumulation of colloid and thyroglobulin within the follicles, thereby reducing hormone release (202). Thyroid volume... 

The thyroid gland has a mass of about 0.025 kg (i.e., a volume of about 25 cm3) therefore, all the beta energr will be deposited in it. 

Thyroid volume, thyroid nodularity, or iodine excretion have close associations to serum Tg, one of the largest iodoglycoproteins in the body with a complex three-dimensional structure, which originates only in the thyroid gland. 

In a number of smaller clinical trials that have been conducted, none of the three modes of medical therapy (iodine monotherapy, levothyroxine monotherapy, or a combination of the two) have been shown to be superior to the others (Hintze and Kobberling, 1992 La Rosa et al., 1995). The maximum benefit to be expected of any medical treatment is a reduction in volume of at most 30% of the total thyroid gland, and even less in that of nodular size. 

The focus here was on the evaluation of the basic characteristics of ioduria and the characteristics of the thyroid (TSH, FT4, antibodies against thyroglobulin and TPO, and the volume of the thyroid gland). 

It is beyond the scope of this review to discuss, except in brief outline, the hypothalamic substance, thyrotropin-releasing hormone (TRH), and its application in clinical chemistry, although many publications on TRH have appeared over the last five years or so. However, only a limited number of clinical applications of TRH are related primarily to the thyroid gland. Some reviews which include information on TRH have appeared recently (H7, H14, S8). A review on hypothalamic releasing hormones appears in this volume (H6A). Reference to the physiological role of TRH has been made in Section 2.4. 

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