Thyroid test revolution: new study seeks better way to monitor hormone levels

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: In order to be eligible to participate in this study, a hypothyroid subject must meet all of the following criteria: * Ability to provide informed consent * Ability to speak and understand Dutch or English * Intake of a stable dosage of levothyroxine, meaning the dosage of levothyroxine must not be changed during the appointment at the outpatient clinic * Diagnosis of one these forms of hypothyroidism * Patients with primary hypothyroidism: euthyroid based on TSH according to physician * Patients with hypothyroidism after a total thyroidectomy due to thyroid carcinoma (therefore athyroid): on target TSH according to physician (target TSH depending on stage/severity of carcinoma) * Patients using L-T4 due to therapy of Graves' disease: euthyroid based on TSH according to physician (TSH cannot be suppressed, namely TSH within reference interval of 0,5-5,0 mU/L) * Patients with central hypothyroidism: euthyroid based on fT4 according to physician (common is fT4 in the upper limit, reference interval is 12-22 pmol/L) In order to be eligible to participate in this study, a healthy control subject must meet all of the following criteria: * Ability to provide informed consent; * Ability to speak and understand Dutch or English * Consider themselves healthy Exclusion Criteria: A potential hypothyroid subject who meets any of the following criteria will be excluded from participation in this study: * Not euthyroid according to physician * Pregnancy * Patients using L-T4 due to therapy of Graves' disease: if TSH is still suppressed * Any of the following medication * Liothyronine (Cytomel) * Iodide * Oral contraceptives * Active treatment of malignancy (other than thyroid carcinoma) A potential healthy control subject who meets any of the following criteria will be excluded from participation in this study: * Pregnancy * Any of the following medication * Thyroid medication (a.o. levothyroxine, thiamazol, PTU) * Lithium * Amiodarone * Propranolol * Iodide * Glucocorticoids * Oral contraceptives * Heparin * Growth hormone * Active treatment of malignancy