Thyroid-stimulating hormone (TSH) is part of a delicate feedback loop designed to maintain optimal levels of circulating thyroid hormones.[1] Traditionally, it is considered the best initial screening marker for thyroid dysfunction for most patients,[2] though additional testing of free thyroid hormones is recommended to provide a clearer diagnostic picture.[3]
Thyroid dysfunction is closely related to depression, the most commonly observed neuropsychiatric symptom in those with hypothyroidism. In view of the crucial role that thyroid hormones play in brain development and function, evaluation of TSH in those with depressive symptoms would be prudent.
Talaei et al. 2017 assess and address depression and hypothyroidism[4]
A cross-sectional study was undertaken of 174 hypothyroid patients 19-68 years old on levothyroxine with TSH levels within the standard lab reference range of 0.5-5 mIU/L. The Beck depression questionnaire was administered and associated with TSH levels.
Overall, depressive symptoms occur in approximately 60% of hypothyroid patients, and severity has been found to correlate with TSH levels. Greater than 95% of a healthy population maintains a TSH level below 2.5 mIU/L, and it is becoming widely accepted that a value within the standard range of 0.5-5 mIU/L is not normal for everyone.
In the study, 116 out of 174 patients were determined to be depressed and had TSH values over 2.5 mIU/L. Levels above 4 mIU/L were associated with severe depression.
Researchers observed that a TSH cut-off of 2.5 mIU/L was able to identify 93.7% of depressed patients in the study and that depressed patients were eight times more likely to have a TSH above 2.5 than hypothyroid cases without depression. A cut-off of 4 mIU/L would be able to detect 82.9% of those with severe depression…a revelation that may save lives.
Considering that the use of thyroid hormones to treat depression has been around since the 1960s, it is not unreasonable to ensure that appropriate levels of thyroid hormone are used to treat depression in known hypothyroid patients. Researchers strongly suggest an optimal cut-off of 2.5 mIU/L to effectively assess and address depression symptoms.
[1]Pirahanchi Y, Jialal I. Physiology, Thyroid Stimulating Hormone (TSH). 2020 Mar 26. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. [R]
[2] Garber JR, Cobin RH, Gharib H,et al; American Association Of Clinical Endocrinologists And American Thyroid Association Taskforce On Hypothyroidism In Adults. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012 Dec;22(12):1200-35. [R]
[3] Ling C, Sun Q, Khang J, et al. Does TSH Reliably Detect Hypothyroid Patients? Ann Thyroid Res. 2018;4(1):122-125. [R]
[4] Talaei A, Rafee N, Rafei F, Chehrei A. TSH cut off point based on depression in hypothyroid patients. BMC Psychiatry. 2017 Sep 7;17(1):327. [R]