Thyroid function in elderly people: The role of subclinical thyroid disorders in cognitive function and mood alterations

Thyroid Hormone Diseases and Osteoporosis

Thyroid hormones are essential for normal skeletal development and normal bone metabolism in adults but can have detrimental effects on bone structures in states of thyroid dysfunction. Untreated severe hyperthyroidism influences the degree of bone mass and increases the probability of high bone turnover osteoporosis. Subclinical hyperthyroidism, defined as low thyrotropin (TSH) and free hormones within the reference range, is a subtler disease, often asymptomatic, and the diagnosis is incidentally made during screening exams. However, more recent data suggest that this clinical condition may affect bone metabolism resulting in decreased bone mineral density (BMD) and increased risk of fracture, particularly in postmenopausal women. The main causes of exogenous subclinical hyperthyroidism are inappropriate replacement dose of thyroxin and TSH suppressive L-thyroxine doses in the therapy of benign thyroid nodules and thyroid carcinoma. Available data similarly suggest that a long-term TSH suppressive dose of thyroxin may decrease BMD and may induce an increased risk of fracture. These effects are particularly observed in postmenopausal women but are less evident in premenopausal women. Overt hypothyroidism is known to lower bone turnover by reducing both osteoclastic bone resorption and osteoblastic activity. These changes in bone metabolism would result in an increase in bone mineralization. At the moment, there are no clear data that demonstrate any relationship between BMD in adults and hypothyroidism. Despite these clinical evidences, the cellular and molecular actions of thyroid hormones on bone structures are not complete clear.

Effect of rhTSH on Lipids

BACKGROUND

Subclinical hypothyroidism is associated with increased blood lipid levels. However, the exact role of thyrotropin (TSH) alone is not clear. In order to clarify this point, we analysed the acute effect of recombinant human TSH (rhTSH) administration on lipid levels.

METHODS

Sera of 27 premenopausal women with well-differentiated thyroid cancer were analysed. Patients that underwent a total thyroidectomy, ablation with ¹³¹I (Iodine 131) and rhTSH administration as a part of routine follow-up American Thyroid Association guidelines were included. The protocol consists of 2 intramuscular injections of 0.9 mg of rhTSH, performed on day 1 day and day 2, with blood collection on day 1 (before rhTSH administration), and day 5. TSH, free thyroxine, total cholesterol, low-density lipoprotein cholesterol (LDLc), high density lipoprotein cholesterol (HDLc), and triglycerides were assessed in all the samples, before and four days after the first administration of rhTSH.

RESULTS

Total cholesterol and triglycerides significantly increased after stimulation of rhTSH (respectively, 192 ± 33 vs. 207 ± 26, p = 0.036 and 72 ± 23 vs. 85 ± 23, p = 0.016). LDLc and HDLc showed comparable concentrations before and after the test (respectively, 115 ± 27 vs. 126 ± 22, p = 0.066, and 62 ± 15 vs. 64 ± 15, p = 0.339), while non-HDLc increased after stimulation (130 ± 30 vs. 143 ± 25, p = 0.045).

CONCLUSION

TSH has a direct effect on total cholesterol, triglycerides, and nonHDLc. Explanation of these phenomena will require additional studies.

Subclinical Hypothyroidism and Lipid Metabolism: To Treat or Not to Treat?

Thyroid hormones have multiple complex effects on lipid synthesis and metabolism. These physiological actions are well documented in overt hypothyroidism where the elevated levels of total cholesterol, low density lipoporotein cholesterol and possibly triglycerides are reverted by levo-thyroxine therapy. Subclinical hypothyroidism, defined as elevated serum thyroid stimulating hormone in the presence of reference range of free thyroxine and free triiodothyronine concentrations, is a relatively frequent clinical conditions. Many clinical and epidemiological studies have evaluated lipid metabolism, markers of subclinical atherosclerosis and other cardiovascular risk factors in subclinical hypothyroidism as well as the need of replacement therapy in these patients. The available results are rather conflicting, with variable and inconclusive results. Moreover, no consensus still exists on the clinical significance and treatment of this mild form of thyroid failure. On the contrary, available evidences suggest that patients with plasma thyroid stimulating hormone levels above 10 mU/L should be treated with levo-thyroxine, since may have an increased risk of cardiovascular disease. However, the epidemiological evidences suggest being rather conservative in older people, since higher thyroid stimulating hormone is associated with lower risk of multiple adverse events in this population. In this review, we summarized the current evidences on the association between subclinical hypothyroidism and lipid metabolism and the effect of levo-thyroxine therapy on lipid parameters.

Subclinical hypothyroidism and cardiovascular risk factors

INTRODUCTION

Thyroid hormones have multiple effects on lipid metabolism as well as on the cardiovascular system function. These negative cardiovascular effects have long been recognized in overt hypothyroidism but can be reversed by treatment with levothyroxine.

EVIDENCES ACQUISITION

We performed on PubMed a literature search for the articles published until March 2019 by using the search terms "subclinical hypothyroidism," "cardiovascular disease," "cholesterol," "LDL," "HDL," "triglycerides," "coronary heart disease," "heart failure," "atherosclerosis," "all-cause mortality," "levothyroxine."

EVIDENCES SYNTHESIS

Subclinical hypothyroidism, defined as an elevated thyrotropin (TSH) with a normal free thyroxine (FT4), is frequent in the general population and increase with age. Subclinical hypothyroidism has been linked to cardiovascular risk factors, dyslipidemia and increased atherosclerosis. Although some studies have demonstrated that lipids are elevated in subclinical hypothyroidism, other studies did not confirm these data. Clinical trials have also demonstrated there is no clear evidence that levothyroxine therapy in subjects with milder form (TSH<10 mU/L) of subclinical hypothyroidism could improve lipid status and the other cardiovascular risk factors. Nevertheless, TSH level seems the best predictor of cardiovascular disease, in particular when its levels are above 10 mU/L.

CONCLUSIONS

Prospective studies are necessary to clarify the cardiovascular risk in patients with mild subclinical hypothyroidism and to assess the importance of treating elderly people in order to improve or counteract the correlated risks. However, until clinical recommendations will be updated, the decision to treat or not treat patients with subclinical hypothyroidism will still base on clinical judgment, clinical practice guidelines, and expert opinion.