Primary hypothyroidism is often detected by elevated TSH values. This is due to the normal feedback-loop of the HPA axis. Central hypothyroidism (CH), however, is not typically detected by measuring TSH which is low due to a disruption of the pituitary's stimulus and is not produced as needed. The most common cause is a pituitary adenoma.
In Mechanisms Related to the Pathophysiology and Management of Central Hypothyroidism, the authors state, "Given that the prevalence of pituitary adenomas in the general population is greater than 10%, the true prevalence of CH might be much higher than that reported". When speaking of the method of measuring TSH levels alone, they say this "approach works, however, only if the hypothalamic-pituitary-thyroid axis is normal. Conversely, the strategy of first-line TSH measurement can miss patients with CH."
The authors give a very nice synopsis of the HPA axis' role in thyroid regulation and function. The diagrams are well-done, also. In Table 1, the causes of CH are listed:
CH usually appears concurrently with other hormone deficiencies. "Hormone deficiencies were seen for luteinizing hormone/follicle-stimulating hormone (LH/FSH) in 85% of patients, growth hormone in 65%, adrenocorticotropic hormone (ACTH) in 62%, TSH in 60%, antidiuretic hormone in 23% and prolactin in 15%."
Table 1. Causes of Central Hypothyroidism
Cause Congenital Acquired Classic causes Space-occupying lesions (brain or pituitary; pituitary adenoma, craniopharygioma, etc.) Yes Yes Radiation No Yes Vascular disease (Sheehan syndrome, etc.) Yes Yes Nonclassic causes Traumatic brain injury or subarachnoid hemorrhage No Yes Drug-induced (bexarotene, carbemazepine, etc.) No Yes Growth hormone therapy No Yes Infection (lymphocytic adenohypophysitis, lymphocytic hypophysitis) No Yes Set point diseases (infant's born to mothers with inadequately controlled Graves disease, etc.) Yes No Genetic mutations Yes No Idiopathic Yes Yes
The authors also detail non-classic causes of CH, including genetic mutations. With one familial case inheritance was determined to be autosomal recessive. Interestingly, the values of TSH measured varied by assay with one mutation.
The researchers make a case for using free T4 and free T3 to determine CH while levels of TSH have no diagnostic value. They also indicate that although the "nocturnal surge of serum TSH level has been used to assess CH, this approach is still controversial. MRI could be required for most suspected cases of CH to detect origin of hypothalamic or pituitary disorders."
Treatment is much the same as for any hypothyroidism. Levoxythyroxine is the first line of treatment. They also carefully point out the following:
Deficiencies of hormones other than TSH should be considered before startingAn algorithm was developed for treating CH sufferers. Interestingly, this does not show using any form of T3, although there are folks who do not convert forms of T4 to the needed T3. They do empasize, however, the importance of achieving "free T4 in the upper end of the normal range rather than within the middle or lower values".
treatment. When ACTH deficiency is also present, glucocorticoid therapy should
be started at least 1 week before initiation of levothyroxine to avoid increased
consumption of cortisol and worsening of the ACTH deficiency, which can
induce crisis. (emphasis mine)
On a positive note for those who have or will undergo pituitary surgery, they authors conclude, "Surgery is reported to lead to an improvement in anterior pituitary function in approximately 35% of patients with pituitary adenoma and CH."
Masanobu Yamada, Masatomo Mori (2008). Mechanisms related to the pathophysiology and management of central hypothyroidism Nature Clinical Practice Endocrinology & Metabolism, 4 (12), 683-694 DOI: 10.1038/ncpendmet0995