Many substances and health factors can, independently of thyroid hormones, lower TSH secretion significantly.
These factors are not listed in Braverman & Cooper’s chapter on thyrotoxicosis in Werner & Ingbar’s The Thyroid textbook, 10th edition 2013, because their chapter avoids defining the syndrome by a low TSH.
Nevertheless, these TSH-lowering factors should always be mentioned along with “thyrotoxicosis” in the context of today’s TSH-worship.
Whenever doctors see low TSH lab results with normal thyroid hormones, they may presume it’s subclinical thyrotoxicosis and that they must do something about it.
Essentially, these are temporary and variable factors that induce “acquired central hypothyroidism” (Beck-Peccoz, 2017; Benvega et al, 2018).
These factors can amplify the negative feedback of thyroid hormones. They compromise the hypothalamus’ ability to secrete TRH hormone and/or the pituitary’s secretion of TSH hormone.
Several of these factors can coexist in the same person. They include, but are not limited to:
Fasting or calorie restriction
Depression, especially with excess Ghrelin
Steroid medications (glucocorticoids) and endogenous cortisol secretion
Inflammatory cytokines (C-Reactive protein)
Low levels of leptin
Reducing a dose of estrogen or progesterone hormone
Narrow vs. wide individual circadian fluctuations in TSH (low in afternoon)
Dopaminergic drugs and Somatostatin analogs
Mitotane, Rexinoids (used in cancer therapy)
(Citations for the above list include Ataoğlu et al, 2018; Beck-Peccoz et al, 2017; Benvega et al, 2018; Cole et al, 2009; Fliers et al, 2014 & 1998; Flier et al, 2000; Jostel et al, 2009; Kluge et al, 2013; Labrie et a, 1978; Russell et al, 2008; Samuels et al, 2000; Sharma et al, 2006)
In general, these factors prove how sensitive TSH is — it’s actually hypersensitive — to the human body’s environment. When conflicting messages occur, the pituitary can get its wires crossed, and TSH can be suppressed by something in addition to thyroid hormones. A patient might actually be nowhere near thyrotoxicosis in their Free T3 and/or Free T4 while their TSH falls lower than it should.
The two best medical resources for the unreliability of a low TSH alone are:
- Chatzitomaris, A., Hoermann, R., Midgley, J. E., Hering, S., Urban, A., Dietrich, B., … Dietrich, J. W. (2017). Thyroid Allostasis–Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming. Frontiers in Endocrinology, 8. https://doi.org/10.3389/fendo.2017.00163
- Haugen, B. R. (2009). Drugs that suppress TSH or cause central hypothyroidism. Best Practice & Research. Clinical Endocrinology & Metabolism, 23(6), 793–800. https://doi.org/10.1016/j.beem.2009.08.003