I wonder if they just assumed that the thyroid system was a thermostat. The analogy is still common today.
While it’s usually a safe bet that a biological system has complexities, it’s entirely possible for it to be a simple control system, where the TSH levels serve as an control signal, and still have a failure mode where TSH is normal or low but thyroid output low.
That possible failure mode is that a problem in the hypothalamus or pituitary could lead to an underproduction of TSH. The properly-functioning thyroid would then dutifully lower production of thyroid hormone to bring TSH back to normal levels.
If this is the case, it wouldn’t be surprising if people with that problem tended to have TSH (on average) on the low side. Why? Because the simplest possible implementation of such a control system at the thyroid end would be to have TSH dictate an “effort” level and with a normal thyroid the effort level corresponding to a low thyroid hormone production would be low.
Simon, failure of the hypothalamus and pituitary is known about, and referred to as ‘central hypothyroidism’, and it does indeed produce the same symptoms but normal TSH values. However you can detect it by other blood tests.
But it’s very rare. So rare that it’s not cost-effective to test for it unless you’ve got some reason to look. Some undiagnosed cases will end up getting labelled as CFS, and that’s a reason to test other hormone levels too if you can’t explain someone’s problems, but it’s not nearly common enough to explain the whole thing (assuming very minimal levels of competence in those investigating).
While it’s usually a safe bet that a biological system has complexities, it’s entirely possible for it to be a simple control system, where the TSH levels serve as an control signal, and still have a failure mode where TSH is normal or low but thyroid output low.
That possible failure mode is that a problem in the hypothalamus or pituitary could lead to an underproduction of TSH. The properly-functioning thyroid would then dutifully lower production of thyroid hormone to bring TSH back to normal levels.
If this is the case, it wouldn’t be surprising if people with that problem tended to have TSH (on average) on the low side. Why? Because the simplest possible implementation of such a control system at the thyroid end would be to have TSH dictate an “effort” level and with a normal thyroid the effort level corresponding to a low thyroid hormone production would be low.
Simon, failure of the hypothalamus and pituitary is known about, and referred to as ‘central hypothyroidism’, and it does indeed produce the same symptoms but normal TSH values. However you can detect it by other blood tests.
But it’s very rare. So rare that it’s not cost-effective to test for it unless you’ve got some reason to look. Some undiagnosed cases will end up getting labelled as CFS, and that’s a reason to test other hormone levels too if you can’t explain someone’s problems, but it’s not nearly common enough to explain the whole thing (assuming very minimal levels of competence in those investigating).