Factors That Can Lower TSH

Aging, infection, trauma, prolonged cortisol excess, somatostatin, dopamine or L-dopa, adrenaline (sometimes; Mannisto, et al., 1979), amphetamine, caffeine and fever can lower TSH, apart from the effect of feedback by the thyroid hormones, creating a situation in which TSH can appear normal or low, at the same time that there is a real hypothyroidism. -Ray Peat, PhD

Acta Endocrinol (Copenh). 1979 Feb;90(2):249-58.
Dual action of adrenergic system on the regulation of thyrotrophin secretion in the male rat.
Männistö, Ranta T, Tuomisto J.
The effect of graded doses of drugs modifying adrenergic activity on basal and cold-stimulated TSH secretion was studied in male rats. alpha-methyl-p-tyrosine (aMPT) (16 h before 30 min cold-exposure), phenoxybenzamine (1 h), Ca-fusarate (1 h) and diethyldithiocarbamate (DDC) (1 and 18 h) dose-dependently depressed the cold-stimulated TSH secretion. The effect of reserpine (24 h) was not significant. Clonidine (1 h), dihydroxyphenyl-serine (DOPS) (1 h), noradrenaline (NA) (1 h), and L-Dopa (1 h) were also effective in decreasing serum TSH levels, but dopamine (DA) (ad 2 mg/kg, 1 h) had no effect. Basal TSH levels were also decreased by various doses of clonidine, DOPS and NA, given ip 1 h before sacrifice. Clonidine (1 mg/kg), NA (1 mg/kg), DA (2 mg/kg), aMPT (300 mg/kg), phenoxybenzamine (2 or 20 mg/kg), Ca-fusarate (50 mg/kg) or L-Dopa (200 mg/kg) did not modify the TRH-induced TSH response. These results cannot be explained by assuming only a stimulatory function for the adrenergic system on the secretion of TSH in the rat. The site of the possible inhibitory function of noradrenaline in the control of TSH cannot be deduced from these results, but various possibilities are discussed.

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