Anxiety, high blood pressure, panic attacks, a fast heart rate, and hypoglycemia can actually be symptoms of low thyroid levels. Thyroid lab results will show low levels of Free T3, Free T4, or both. The anxiety, tachycardia (fast heart rate), and high blood pressure these people experience is not from being hyperthyroid or overmedicated, but from noradrenaline that the body is secreting for energy to compensate for the lack of thyroid hormone. Unfortunately, they may be prescribed blood pressure medications (such as beta blockers) and/or anti-anxiety medications (such as benzodiazepines). Neither of these pharmaceutical prescriptions corrects the low thyroid condition that caused the symptoms in the first place, and both have side effects. In one study, noradrenaline was three times higher in hypothyroid subjects than normal controls when lying down.  This may explain why my heart always felt like it was racing whenever I laid down–but couldn’t fall asleep. I had RAI so cannot possibly be hyperthyroid unless I take an overdose of thyroid medication. When I was only taking ½ grain of desiccated thyroid and obviously undermedicated, my pulse was 100 and my BP was 170/100. Raising to 2 grains brought the pulse back into the 80s and BP down to about 130/80. This is completely counterintuitive, which is why it’s so difficult to understand. Thyroid and noradrenaline (norepinephrine) have an inverse relationship. [1- 4] In other words, as thyroid levels rise, norepinephrine decreases, and vice versa. Norepinephrine is one of the hormones involved in the “fight or flight response” that causes hyperarousal.
When someone with these symptoms tries to raise their dose just a hair, by ¼ grain or 25 mcg of T4, they get hyperthyroid symptoms, so return to their previous dose and claim that must be the correct dose. This intolerance to thyroid hormone may be due to low cortisol and/or low iron/ferritin. If both iron and ferritin are low (usually less than 70 ng/mL but everyone’s threshold is different), taking thyroid hormone, especially any with T3, will cause intolerable anxiety, and again, they will return to their previous lower dose. [7,8,9] They should return to their previous dose, but also address the low cortisol and low iron problems with supplements, if lab tests confirm that they are truly low. [thyroid metabolism requires iron and cortisol] Hyperthyroidism causes many negative psychiatric symptoms, but hypothyroidism can cause depression, panic attacks, auditory and visual hallucinations, and paranoid delusions. [10,11]
An insatiable appetite can be either a hyperthyroid or hypothyroid symptom. In the hyperthyroid state, high T3 levels raise the metabolism, and increased food intake is required to meet that demand. In the hypothyroid state, extremely low cortisol can result from extremely low thyroid levels. Thyroid and cortisol usually rise and fall in tandem.  The low cortisol results in severe hypoglycemia, which keeps triggering the body to eat, to keep blood glucose up. I had to eat every 1.5 – 2 hours when I was in this state (even in the middle of the night), lest I have another hypoglycemic attack, which was like a panic attack: my heart would suddenly race, I’d get very hot and break out in a sweat, my whole body would shake, I would lose control of my hands (computer mouse would be released), and then it would all just stop. This is referred to as a hypoglycemic seizure. I no longer have them now that I’m on a higher dose of thyroid medication. I lost a considerable amount of weight while taking only 1/2 grain, and I looked gaunt. So yes, someone who is severely hypothyroid can actually be thin. Now that I’m taking significantly more thyroid medication, I actually eat less and have regained 20 pounds, but it’s a healthier look.
Low cortisol as a cause of hypoglycemia is illustrated in a case study of a 7-year-old girl with zero cortisol. She had a genetic ACTH deficiency, and her plasma ACTH was undetectable. (ACTH is a pituitary hormone that signals the body to release cortisol.) She had severe hypoglycemia, was admitted to the hospital unconscious, and regained consciousness after treatment with intravenous glucose. Her daily treatment now consists of oral hydrocortisone in three divided doses; this has resolved her symptoms and normalized her blood glucose. 
Hyperpigmentation, or the darkening of the skin in certain areas (armpits, anogenital area, gums) results from the high ACTH that tells the adrenals to produce more cortisol. Hyperpigmentation is found in 95% of patients with Addison’s disease or chronic primary adrenal insufficiency. However, someone can have very low cortisol and have normal looking skin if their ACTH is low due to pituitary dysfunction rather than an underperforming adrenal gland. My darkened skin areas have lightened considerably since switching from 100% desiccated to mostly T4 with a little desiccated. I can only guess that lowering the T3 resulted in the need for less cortisol, and therefore, my ACTH has decreased. 
Getting your thyroid tested
If you’d like to have your thyroid levels tested, please ask for these thyroid tests, and note where your levels are in the thyroid lab ranges compared to healthy people. If you do not ask for these specific tests, your doctor will most likely just run a TSH test, which sadly, does not catch many cases of hypothyroidism. [TSH levels do not reflect thyroid levels]
- J. Faber, L. Petersen, N. Wiinberg, S. Schifter, J. Mehlsen. Hemodynamic Changes After Levothyroxine Treatment in Subclinical Hypothyroidism. Thyroid. April 2002, Volume: 12 Issue 4, 319-324 http://www.ncbi.nlm.nih.gov/pubmed/12034057?dopt=Abstract
- Enza Fommei and Giorgio Iervasi. The Role of Thyroid Hormone in Blood Pressure Homeostasis: Evidence from Short-Term Hypothyroidism in Humans. The Journal of Clinical Endocrinology & Metabolism Vol. 87, No. 5 1996-2000, 2002. http://jcem.endojournals.org/cgi/content/full/87/5/1996
- P. Manhem, B. Hallengren, B.G. Hansson. Plasma Noradrenaline And Blood Pressure In Hypothyroid Patients: Effect Of Gradual Thyroxine Treatment. Clinical Endocrinology. Volume 20, Issue 6, pages 701–707, June 1984. http://www.ncbi.nlm.nih.gov/pubmed/6467635
- Levey GS, Klein I. Catecholamine-thyroid hormone interactions and the cardiovascular manifestations of hyperthyroidism. Am J Med. 1990 Jun;88(6):642-6. http://www.ncbi.nlm.nih.gov/pubmed/2189309
- Sonia C Dumoulin, Bertrand P Perret, Antoine P Bennet and Philippe J Caron. Opposite effects of thyroid hormones on binding proteins for steroid hormones (sex hormone-binding globulin and corticosteroid-binding globulin) in humans. European Journal of Endocrinology, Vol 132, Issue 5, 594-598,1995.http://www.eje-online.org/cgi/content/abstract/132/5/594
- Michael Y. Torchinsky, Robert Wineman, and George W. Moll, “Severe Hypoglycemia due to Isolated ACTH Deficiency in Children: A New Case Report and Review of the Literature,” International Journal of Pediatrics, vol. 2011, Article ID 784867, 3 pages, 2011. http://www.hindawi.com/journals/ijped/2011/784867/cta/
- Jonathan Stephen Murray, Rubaraj Jayarajasingh, Petros Perros. Deterioration of symptoms after start of thyroid hormone replacement. BMJ 2001; 323 : 332. http://www.bmj.com/content/323/7308/332.1.extract
- I A Osman, Peter Leslie. Adrenal insufficiency should be excluded before thyroxine replacement is started (Letter to the Editor). BMJ Volume 313, 17August 1996. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2351829/pdf/bmj00555-0059b.pdf
- K M Shakir, D Turton, B S Aprill, A J Drake, 3rd and J F Eisold. Anemia: a cause of intolerance to thyroxine sodium. Mayo Clinic Proceedings. February 2000 vol. 75 no. 2 189-192. http://www.mayoclinicproceedings.com/content/75/2/189.full.pdf+html
- Mitsuru Kikuchia, Ryutarou Komurob, Hiroshi Okac, Tomokazu Kidania, Akira Hanaokaa, Yoshifumi Koshino. Relationship between anxiety and thyroid function in patients with panic disorder. Progress in Neuro-Psychopharmacology & Biological Psychiatry 29 (2005) 77– 81. http://www.scivac.it/sisca/pdf/2005/panico_tiroide220305.pdf
- Thomas W. Heinrich and Garth Grahm. Hypothyroidism Presenting as Psychosis: Myxedema Madness Revisited. Primary Care Companion. Journal of Clinical Psychiatry. 2003; 5(6). http://www.psychiatrist.com/pcc/pccpdf/v05n06/v05n0603.pdf
- Elizabeth A Liotta, Dirk M Elston, Alexander Brough. Addison Disease Clinical Presentation: Physical. Medscape.
- Christensen, Niels Juel. Increased levels of plasma noradrenaline in hypothyroidism. Journal of Clinical Endocrinology & Metabolism 35.3 (1972): 359-363.