Brief Answer:
Thyroid

Detailed Answer:
Sorry to learn about your borderline TSH value.

To prevent becoming hypothyroid, there are couple of things to keep in mind.

Adequate amounts of iodine in your diet are necessary for normal thyroid function. If you live in a country which follows a mandatory National Salt Iodization program, then it is extremely unlikely you are iodine deficient. Salt Iodization implies compulsory addition of Iodine to salt by all salt manufacturers of the country.
So unless you strictly avoid all salt in cooking at home, and all food consumed outside, it is not possible for you to be getting insufficient amounts of iodine.

The other thing is 'cruciferous vegetables' like cabbage, cauliflower and broccoli. These, when consumed in very large quantities, can hamper the thyroid. Please note, for all practical purposes this is hard to achieve ie one would have to eat a lot of these daily for it to adversely affect thyroid function. So in fact I actually encourage my patients to eat these in good measure because they represent healthy food items.
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)

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