This topic assumes you have some understanding of the thyroid and how thyroid hormones are regulated. If not you will need to read up. The Society for Endocrinology gives a good description of the thyroid.
The term ‘hypothyroid’ is used to define patients with signs and symptoms of hypothyroidism, regardless of their blood hormone levels. Hypothyroidism can only be definitively diagnosed by its presenting signs, symptoms and response to treatment.
There are a number of websites with some unhelpful and inaccurate content which should be ignored. Endocrinologists, alternative doctors and some patients seem to be in competition to provide the most misleading information. For example, ‘you can’t be hypothyroid with normal hormone levels’, or need ‘high iron levels‘ for thyroid hormone to ‘work‘. Take a common sense approach, ask simple questions. Does it makes sense? Has the author studied the subject in depth? Are they giving hundreds of vague references to papers? Is the website set up for marketing, or to protect vested interests? Check everything.
Primary hypothyroidism is when the thyroid gland cannot secrete the normal amount of hormone. Often when doctors refer to hypothyroidism they are (only) thinking of primary hypothyroidism. It is by far the most common form of hypothyroidism arising from insufficient secretion.
Secondary and Tertiary Hypothyroidism
Secondary hypothyroidism is when the pituitary fails to produce sufficient TSH to stimulate the thyroid. Tertiary hypothyroidism is when the hypothalamus secretes insufficient TRH to stimulate the pituitary to produce enough TSH. These are collectively referred to as ‘central hypothyroidism’. They are rare conditions that require specialist investigation. They are usually detected when thyroid hormones are low and TSH is low.
Other Types of Hypothyroidism
There are other causes of insufficient thyroid hormone activity that are recognised. These include some rare genetic disorders such as mutations of the gene for thyroid hormone transport into cells (MCT8) and genetic resistance to thyroid hormone (RTH). The cruical point is that these are conditions that can be detected by abnormal blood or genetic tests. Hypothyroidism with normal blood test results is deemed to be impossible and hence it is not recognised or looked for.
Hypothyroidism with normal blood test results exists and is common.