This is the best explanation about Thyroid Hormone Resistance I have found so far.
Dr. Lowe gave this answer:
When supraphysiologic dosages of thyroid hormone fail to suppress TSH secretion, the patient has "general" resistance to thyroid hormone. This classification of thyroid hormone resistance is different from the one that manifests as fibromyalgia in many patients. The latter classification is termed "peripheral" resistance to thyroid hormone. I will explain the difference between general and peripheral resistance.
The classification of thyroid hormone resistance is based on whether (1) the pituitary gland and (2) the other tissues of the body (referred to as "peripheral" tissues) are involved in the patient's resistance. In general resistance, both the pituitary gland and the peripheral tissues are partially resistant to thyroid hormone. The patient's thyroid hormone levels are elevated, but his TSH level and his peripheral tissue metabolism are usually normal. Here's why:
In normal functioning of the hypothalamic-pituitary-thyroid axis, when the amount of thyroid hormone in the blood reaches an appropriate level, it then inhibits secretion of TSH by the pituitary gland (this is a normal "feedback" mechanism). But, when the pituitary tissue is resistant to thyroid hormone, much larger amounts are necessary to suppress TSH secretion. When the peripheral tissues are also partially resistant, large amounts of thyroid hormone are needed to "override" the resistance and drive the subnormal metabolism in the periphery to a higher, normal rate. Typically, when these large amounts of thyroid hormone (secreted by a thyroid gland stimulated by large amounts of TSH from a resistant pituitary) finally increase enough to normalize the metabolic rate of the patient's peripheral tissues, the pituitary tissue also finally responds to this amount by reducing its secretion of TSH into the normal range. Thus, patients with general resistance usually have clinically normal peripheral tissue metabolism with high levels of thyroid hormone in the blood, and a normalized level of TSH (finally suppressed only by the high blood levels of thyroid hormone). In this way, the circulating levels of thyroid hormone are kept high enough to override the peripheral tissue resistance (with normalized metabolism) and the pituitary resistance (with normalized TSH). In other words, many general resistance patients appear clinically normal except for the high thyroid hormone levels.
In peripheral resistance to thyroid hormone, only the peripheral tissues are resistant. The pituitary tissue responds normally to a normal amount of thyroid hormone in the blood, and it decreases its TSH output when the blood levels signal that it should do so. Normal blood levels of thyroid hormone, then, properly suppress pituitary gland release of TSH, and keep the circulating TSH levels normal. What's important to realize in this scenario is that the feedback between the pituitary gland and the thyroid gland is normal, and both glands secrete normal amounts of their respective hormones. But the normal thyroid hormone levels are insufficient to override the partial resistance of tissues other than the pituitary--that is the peripheral tissues. As a result, metabolism in the peripheral tissues is subnormal. To speed peripheral tissue metabolism up to normal, the peripheral resistance patient must use large dosages of thyroid hormone. But the effect of these large dosages on the normally responsive pituitary tissue is suppression of the TSH. Thus, secretion of TSH, and its circulating level, are extremely low.
Unfortunately, most physicians become alarmed when they measure the TSH level in such a patient and find it extremely low. Physicians have been taught that a low TSH level means only one thing in a patient taking thyroid hormone--the dosage is too high and is going to harm the patient. It will take some years for physicians to learn about peripheral resistance to thyroid hormone and to understand the odd test values these patients have when taking effective dosages of thyroid hormone. Patients with peripheral resistance must take TSH-suppressing dosages of thyroid hormone to have normal peripheral tissue metabolism. But there is nothing whatsoever harmful to these patients in having their TSH suppressed by these dosages of thyroid hormone. In fact, it would be harmful for most of them not to take such dosages. The adverse consequences include conditions such as fibromyalgia, chronic fatigue syndrome, and liver and cardiovascular diseases.
To see the entire post go here:
http://www.drlowe.com/QandA/askdrlowe/resistnc.htm
No comments:
Post a Comment