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It is a well-known fact that in patients with pituitary insufficiency
and hypothyroidism serum TSH is often in the normal range or even elevated.
The following report addresses the question if this hypothyroidism is due
to either reduced bioactivity of TSH or reduced responsiveness of the thyroid
to TSH. Horimoto et al., Osaka and Kobe, Japan, studied in
7 patients with central hypothyroidism the ratio bioactivity/immuno reactivity
of TSH as reflected by the _ T3 versus _ TSH after TRH. Compared with normal
controls no difference in ths ratio was found between the mean values in
the two groups. The central hypothyroidism was reflected by low total and
free T4 levels in the presence of documented deficiency of anterior pituitary
hormones other than TSH. Since in these patients TSH varied between 2.2
to 14.8mU/L, it was concluded that hypothyroidism was due to decreased
thyroidal response to TSH. (Comment: This is an interesting finding, however
the results are obtained in a limited number of patients. If these studies
are confirmed in a larger group of patients, the results suggest that the
deficiency of anterior pituitary hormones results in the lowered responsiveness
of the thyroid to apparently normal TSH. It should however be remembered
that there are several reports in the literature suggesting that TSH of
patients with pituitary tumors and resulting pituitary insuffiency has
lowered bioactivity as a result of a change in glycosylation of the TSH
molecule.) Whether subclinical hypothyroidism (SCH) (elevated TSH with
normal free T4 and free T3, and clinical euthyroidism) is an increased
risk factor for cardiovascular disease (CVD), is still a matter of debate.
Ladenson, Baltimore, MD, USA, investigated thyroid function in 3,410 subjects over 65 years. They found that in euthyroid patients and SCH patients increasing TSH was related to higher total cholesterol, LDL-cholesterol, triglycerides and factor 7. However, there were no differences between euthyroid and SCH patients with regard to symptomatic or clinical signs for CVD. It is concluded that, although in SCH risk factors for CVD are more common, prevalence of CVD is not higher in patients with SCH. It is suggested that longitudinal follow-up is necessary to define the actual risk of CVD in subjects with SCH. Ain, Lexington, KY, USA, studied brain phosphorus metabolism in acute hypothyroidism in 9 adult patients after 6weeks withdrawal of T4 post-surgery for thyroid carcinoma and compared the results after adequate T4 substitution. They found decreased phosphocreatinine/inorganic phosphate and increased Pi/ATP ratios after T4 substitution indicating similar changes in phosphate metabolism in the brain, as found in skeletal muscle during hypothyroidism. It is stated that this study provides the first direct evidence for metabolic effects of acute hypothyroidism on adult human brain. |
