All patients diagnosed with HT in the study were receiving LT4 treatment. In the PMBT group, in addition to thyroid hormone replacement therapy, a continuous wave GaAIAs type diode laser device (Intelect® Mobile Laser, Model No: 2779, Production Year: 2016; Chattanooga Group) was also applied to the thyroid gland in a 0.07 cm² treatment area. Continuous mode at 850 nm wavelength, 100 mW output power, 1.43 W/cm2 power density and 28.57 J/cm2 energy density (28.57 J/cm2 =100 Mw x 20 s/0.07) were used during the photobiomodulation therapy.

In the sham PMBT group, the probe was placed similarly to the treatment group. The screen of the laser device was active; however, the energy was set to 0 J and the power to 0 Mw, and the same procedures were also performed.

Before each treatment session, the borders of the thyroid gland were marked with ultrasound by an endocrinology specialist. Eight target points (superior, mediolateral, inferior borders of the right and left thyroid lobes, right and left side of the isthmus) were demarked with a surgical pen. PMBT was applied to the marked areas on the thyroid gland, which were approximately 1 cm apart from each other.

Fatigue level in all patients was assessed by Fatigue Impact Scale (FIS) and Fatigue Severity Scale (FSS). The FIS is a forty-item scale indicating the consequences of fatigue on daily life such that it includes cognitive, physical and psychosocial subscales. The sum of the subscales ranges from 0 to 40 (physical and cognitive) and 0-80 (psychosocial); a higher score indicates a more severe impact of fatigue in daily life. FSS assesses the degree of physical fatigue over the past week and includes nine items. Patients indicate how much they agree or disagree with the statements for each item on a seven-point Likert scale. FSS scores ≥ 4.0 indicate clinically relevant fatigue.

General fatigue feeling of the individuals were evaluated by Visual Analogue Scale (VAS).Individuals were asked to mark their fatigue level on the scale. '0' means 'not tired at all' and '10'means 'very tired'.

The Pittsburgh Sleep Quality Index (PSQI) was used to assess sleep quality based on participants' sleep experiences in the past week. The PSQI is a self-administered questionnaire that has been proven to be a valid and reliable tool for measuring sleep quality and quantity. The PSQI consists of a total of seven components and nineteen questions, including subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medication and daytime dysfunction. The sum of the seven dimensional components gives global scores ranging from 0 to 21, with higher scores indicating poorer sleep quality. Those having score above 5 are considered as to have "poor sleep quality" and those having score 5 and below are considered as to have "good sleep quality" .