Environmental factors can affect thyroid hormone levels and activity. For example, thyroid hormone synthesis requires iodine and amino-acids, especially tyrosine, as building blocks while selenium is required for deiodinases, so deficiency may cause hypothyroidism.[⁵]

Conversely, excess iodine from diet, dietary supplements, or some medications containing iodine or iodides may stimulate hyperthyroid symptoms. Calcium and iron can reduce absorption of levothyroxine.[²⁸]

Other hormones can also affect thyroid hormone levels. Examples include somatostatin (growth inhibiting hormone), glucocorticoids and dopamine which can inhibit TSH production. Cold, stress, and exercise can increase TRH release.[⁴]

Thyroid UK has a comprehensive list of medicines (https://thyroiduk.org/food-and-drug-interactions/) which can adversely affect thyroid activity, trigger thyroid conditions, or interact with thyroid medication. This can include by affecting levothyroxine absorption, increasing or decreasing T4/T3 levels by affecting degradation or transport protein binding, or modifying TSH release.[²⁸]

For example, drugs such as propylthiouracil, dexamethasone, amiodarone and propranolol can inhibit deiodinases reducing T4 to T3 conversion. Amiodarone can disturb thyroid activity in up to 10% of users, either increasing or decreasing thyroid activity. Lithium may trigger Hashimoto’s disease.[^4,15,28,29]

Decongestants, especially sympathomimetics such as oxymetazoline, should be used with caution in hyperthyroidism. Diabetes patients already on insulin or oral hypoglycaemics and needing to start thyroid replacement therapy should be closely monitored as levothyroxine doses may not be as effective as anticipated.[^28,30]