The thyroid and its control.

In considering thyroid hormone regulation and homeostasis, it is important to examine some characteristics of this hormone in relation to the others. The hor­ mones may readily be divided into two distinct physicochemical and biological classes. (Table 1). The protein and peptide hormones are water soluble and evidently exist in the plasma without demonstrable interaction with other serum proteins. These hormones have a rapid metabolic turnover with a half-time in minutes. Therefore it is not surprising to observe rather striking fluctuations in concentration in the blood within a few minutes in the case of all these peptide hormones, including parathyroid hormone, thyrocalcitonin, growth hormone, FSH, LH, prolactin, ACTH, MSH, TSH, vasopressin, insulin, secretin, angiotensin II, and glucagon (Table 1). Indeed, the amino acid hormones, epinephrine and norepinephrine, may fluctuate markedly within a few seconds. In contrast, the "target" hormones--cortisol, progesterone, estradiol, testoster­ one, and thyroxine-are relatively hydrophobic and exist in aqueous solution in the blood by virtue of their firm binding by one or more serum protein carriers (Table 1). The biologic half-times vary with the magnitude of the "free" or nonprotein­ bound moiety. Thus cortisol, which is about 95% protein bound and 5% unbound, has approximately a one-hr half-time, and thyroxine, more than 99.96% protein bound and less than 0.04% unbound, has a biologic half-time of turnover approx-