Hyperthyroidism causes sympathetic overactivation such that many of the symptoms of thyroid storm can be alleviated by beta-blockers. The sympathetic nervous system “fright, flight or fight” response opposes the parasympathetic nervous system “rest and digest” response and shuts down gastrointestinal function. So hyperthyroidism causes constipation, correct? 

Sorry, not correct. Yes, hyperthyroidism can stimulate overactivation of the sympathetic nervous system. Yes, symptoms of thyroid storm can be treated with sympatholytic beta-blockers. But no, hyperthyroidism does not cause constipation. Hyperthyroidism causes diarrhoea.  Conversely, hypothyroidism causes constipation.

So, next, you will ask “What is the mechanism?”. Unfortunately, the mechanism is not known. Recent reviews have speculated that it might be due to beta-2 adrenoceptor-mediated effects on gastrointestinal motility and secretions (Daher et al., 2015; Kyriacou et al., 2015) but the evidence for this is very limited.  For example, a case report on one patient has suggested that propranolol can control intractable diarrhoea in hyperthyroidism (Bricker et al., 2001) but another study on ten hyperthyroid patients found no effect of propranolol on the gastrointestinal transit time (Bozzani et al., 1985).

For the moment, as we do not know the underlying mechanism, it is just one of those exceptions that you have to remember. In nearly every other respect, hyperthyroidism has a sympathomimetic effect and hypothyroidism has a sympatholytic effect. But for the gastrointestinal system, it is the opposite.

References:

Bozzani A, Camboni MG, Tidone L, Cesari P, Della Mussia F, Quatrini M, Ghilardi G, Ferrar L, Bianchi PA (1985) Gastrointestinal transit in hyperthyroid patients before and after propranolol treatment. Am J Gastroenterol. 1985 Jul;80(7):550-2.

Bricker LA, Such F, Loehrke ME, Kavanaugh K (2001) Intractable diarrhea in hyperthyroidism: management with beta-adrenergic blockade. Endocr Pract. 2001 Jan-Feb;7(1):28-31.

Daher R, Yazbeck T, Jaoude JB, Abboud B (2009) Consequences of dysthyroidism on the digestive tract and viscera. World J Gastroenterol. 15(23):2834-8.

Kyriacou A, McLaughlin J, Syed AA (2015) Thyroid disorders and gastrointestinal and liver dysfunction: A state of the art review. Eur J Intern Med. 26(8):563-71.