Brief Answer:
Allergy

Detailed Answer:
Sorry to learn about your bothersome symptoms.

Sometimes , people get an allergic reaction to the XXXXXXX used to color the synthroid tablet. In such situations I ask my patients to try the colorless white pill of 50 mcg which has no XXXXXXX in it. Of course they must take it in a dose equivalent to theirs so for example 2 and quarter tablets of 50 mcg would equate 112. I understand that is challenging btu at least it aids in knowing whether it is the XXXXXXX in the pill which is responsible for the reaction.

However, it is a better idea to see the family doctor or dermatologist first because the allergy described above is highly uncommon.
When I see someone like you in my practice, I typically order the following blood tests in addition to a detailed physical examination:

CBC (Complete Blood Count, also known as Hemogram; includes Hemoglobin, WBC and Platelet counts)
Electrolytes (Sodium and Potassium in particular)
HbA1c (Glycosylated Hemoglobin = your last 3 months' glucose average). Also known by other names such as GlycoHemoglobin or Glycated Hemoglobin or A1c
Liver function tests (SGOT , SGPT, Albumin, Bilirubin, Alkaline Phosphatase)
Kidney function tests (BUN, Creatinine)
TSH (checks your thyroid)
Free T4 (this too checks your thyroid) Thyroid autoantibodies : Anti Thyroid Peroxidase (TPO) antibodies and Anti Thyroglobulin antibodies
25 hydroxy Vitamin D levels (ideal range 40 to 60 ng/ml = 100 to 150 nmol/liter)

None of these tests require any fasting and can be done at any time of the day Correct diagnosis and treatment requires the opportunity to examine the patient so you must see an endocrinologist in-person.

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