Biochemical Testing of the Thyroid: TSH is the Best and, Oftentimes, Only Test Needed – A Review for Primary Care

Abstract

Disorders of thyroid function are common, and screening, diagnosis, and management are often performed by primary care providers. While management of significant biochemical abnormalities is reasonably straight forward, laboratory tests only slightly outside, or even within, the normal range are becoming more difficult to appropriately manage. A large part of this increasing difficulty in appropriate management is caused by patients requesting, and even demanding, certain tests or treatments that may not be indicated. Symptoms of thyroid dysfunction are non-specific and extremely prevalent in the general population. This, along with a growing body of information available to patients via the lay press and internet suggesting that traditional thyroid function testing is not reliable, has fostered some degree of patient mistrust. Increasingly, when a physician informs a patient that their thyroid is not the cause of their symptoms, the patient is dissatisfied and even angry. This review aims to clarify the interpretation of normal and mild abnormalities of thyroid function tests by describing pituitarythyroid physiology and through an in depth review of, arguably, the three most important biochemical tests of thyroid function: TSH, free T4, and anti-TPO antibodies. It is important for primary care providers to have an understanding of the shortcomings and proper interpretation of these tests to be better able to discuss thyroid function with their patients.

Cigarette Smoke Extract Disrupts Transcriptional Activities Mediated by Thyroid Hormones and Its Receptors

“…Cigarette smoking has been related to thyroid disease, although studies on the same have not reported consistent results, with some studies linking smoking to increased TH levels and others to decreased TH levels. [15][16][17][18][19][20][21][22][23] Thyroid dysfunction and cigarette smoking are both common in the general population, with a prevalence of approximately 10 and 30%, respectively. 39) These epidemiological studies presumed a direct effect of the constituents of cigarette smoke on the thyroid gland, as some components of tobacco smoke have been reported to interfere with thyroid function.…”

“…Depending on the reports, alterations in thyroid-stimulating hormone (TSH) and/or TH levels were different from each other. [15][16][17][18][19][20][21][22][23] Because these alterations are usually mild, the effect of smoking on thyroid function is still controversial and its effect on thyroid hormone action is unknown.…”

“…[15][16][17][18][19][20][21][22][23] Smoking seems to induce changes in thyroid function. Depending on the reports, alterations in thyroid-stimulating hormone (TSH) and/or TH levels were different from each other.…”

A Correlation Analysis of Serum Creatinine Based eGFR and Serum Cystatin C Based eGFR with Thyroid Dysfunction Patients

“…Therefore, when assessing kidney function of patients with overt hyperthyroidism, cystatin C-based eGFR can be described as an indicator that more accurately reflects www.kjcls.org When measuring GFR of patients with overt hyperthyroidism, serum creatinine-based eGFR is affected by T 3 , T 4 and TSH and is not affected by free T 4 . On the other hand, cystatin C-based eGFR is not affected by thyroid hormone and TSH.…”

“…In overt hypothyroidism, renal perfusion is affected by hemodynamic changes due to decrease of renal blood flow (RBF), decrease of cardiac output, decrease of renin angiotension system (RAS) activity, decrease of atrial natriuretic factor (ANF), etc., resulting in decrease of glomerular filtration rate (GFR). Conversely, in overt hyperthyroidism, blood volume and glomerular filtration pressure increase due to increase of cardiac output, RAS activation and increase of RBF, resulting in increase of GFR [4][5][6][7][8]. Best known method of measuring GFR is to directly administer external markers like Inulin, Iohexol, Due to difficulty of having to directly administer markers on the patient, GFR is evaluated quickly and easily using a formula based on internal markers like serum creatinine, cystatin C, etc [9][10][11].…”