After obtaining informed consent we will enroll patients with autoimmune hypothyroidism and persistent tiredness despite normalized TSH levels on LT4 monotherapy. A general physical examination will be performed, and in- and exclusion criteria will be checked. This also includes an ECG to prevent including patients with a functional or structural abnormal heart. The study will start with a run-in period, during which all patients switch to blinded generic LT4, which is produced and distributed by the trial pharmacy. This is because there are seven LT4 preparations available in the Netherlands with different pharmacokinetic properties, which would otherwise introduce substantial bias. Previous research has shown that 36% of patients need dose adjustments when switching to other LT4 preparations. Therefore, serum TSH levels are measured every 8 weeks, and medication dosages adjusted if needed, in order to obtain normal serum TSH levels, defined as TSH levels within the assay-specific reference range. For trial feasibility, patients will be excluded from this study and referred back to their referring physician when a normal TSH cannot be reached with a maximum of two dose adjustments. Once a normal TSH has been measured, the final run-in TSH measurement will be performed 8 weeks later. This is because we want to ensure that we only enroll patients with a stable (i.e. normal) TSH on a stable dose of generic LT4, as recent dose adjustments could otherwise impact the tiredness questionnaire scores at the start of the RCT. This TSH measurement will be combined with a repeat ECG and the patient will be asked whether tiredness with a large negative impact on daily life is still present. Patients will enter Stage 2 (RCT) when they have a normal TSH, no ECG abnormalities, and indicate they have persistent tiredness. Patients not fulfilling these criteria will be excluded from this study and referred back to their referring physician. The expected duration of the run-in period will be 4-8 months, depending on the number of dose adjustments.

Stage 2 includes a 1-year double-blind randomized placebo-controlled trial comparing LT4/LT3 with LT4/placebo treatment. At baseline, patients are randomized to either LT4/LT3 or LT4/placebo treatment. Serum TSH levels are measured at every visit, and medication dosages adjusted if needed, with the goal to normalize serum TSH levels, defined as TSH levels within the assay-specific reference range. Patients are excluded and referred back to their referring physician in case a normal TSH cannot be reached with the available study medication dosages (62,5 - 237,5 µg LT4 monotherapy dose). Visits will take place at baseline, and weeks 8, 16, 26, 39 and 52. The density of visits is higher at the beginning as this is the period when dose adjustments are particularly expected. At every visit, patients are asked to complete questionnaires on thyroid related complaints and quality of life (ThyPRO), general quality of life (EuroQoL-5D-5L and EuroQoL-5D-VAS), medical consumption (iMCQ) and productivity losses (iPCQ). At baseline and 52 weeks, additional blood will also be drawn to determine genetic variants, (thyroid hormone) metabolites and collect material for biobanking, as these data will be key to identify subgroups who are likely to respond to LT4/LT3 combination therapy. Although we do not expect any detrimental effects of these low physiological LT3 dosages, and previous studies neither reported any detrimental effects, we will explore the effects of LT4/LT3 combination therapy on the most important thyroid hormone target organs (bone, cardiovascular, metabolic and brain). This will improve communication between medical professionals and patients, providing them with comprehensive information on the balance between the potential benefits and harms of combination therapy vs monotherapy and will enable shared-decision making that better addresses individual needs of patients. For bone, serum bone markers will be determined at baseline and 52 weeks, and DXA scans will be performed at baseline and 52 weeks in a subgroup of 200 individuals. For cardiovascular and metabolic endpoints, blood pressure, pulse rate, weight, and waist circumference are measured at every visit. Fat percentages will be assessed on the beforementioned DXA scans. A repeat ECG will be performed at 52 weeks. To objectively explore effects on neurocognitive function, next to the subjective ThyPRO questionnaire scores, neurocognitive tests will be performed in a subgroup of 200 patients at baseline and 52 weeks.